Use the code below to add the "add buton" to a sonata model autocomplete form Raw

ProductAdmin.php

<?php
// src/ACSEO/AdminBundle/Admin/ProductAdmin.php

// ...

    protected function configureFormFields(FormMapper $formMapper)
    {
      // ...
      $formMapper
        ->add('codes', 'sonata_type_model_autocomplete', array(
          'property' => "name",
          'multiple' => true,
          'template'=>'ACSEOAdminBundle:Form/Type:sonata_type_model_autocomplete_add.html.twig'
      ))
      // ...

sonata_type_model_autocomplete_add.html.twig

{#
file src/ACSEO/AdminBundle/Resources/views/Form/Type/sonata_type_model_autocomplete_add.html.twig

This file is part of the Sonata package.

(c) Thomas Rabaix <thomas.rabaix@sonata-project.org>

For the full copyright and license information, please view the LICENSE
file that was distributed with this source code.

#}
{% spaceless %}

    <input type="text" id="{{ id }}_autocomplete_input" value=""
        {%- if read_only %} readonly="readonly"{% endif -%}
        {%- if disabled %} disabled="disabled"{% endif -%}
        {%- if required %} required="required"{% endif %}
    />

    <div id="{{ id }}_hidden_inputs_wrap">
        {% if multiple -%}
            {%- for idx, val in value if idx~'' != '_labels' -%}
                <input type="hidden" name="{{ full_name }}[]" {%- if disabled %} disabled="disabled"{% endif %} value="{{ val }}">
            {%- endfor -%}
        {% else -%}
            <input type="hidden" name="{{ full_name }}" {%- if disabled %} disabled="disabled"{% endif %} value="{{ value[0]|default('') }}">
        {% endif -%}
    </div>
    
    {# This part is a copy / paste  from sonata-project/doctrine-orm-admin-bundle/Resources/views/CRUD/edit_orm_many_to_one.html.twig #}
    
    {% if sonata_admin.edit != 'admin' and sonata_admin.field_description.associationadmin.hasRoute('create') and sonata_admin.field_description.associationadmin.isGranted('CREATE') %}
        <a  href="{{ sonata_admin.field_description.associationadmin.generateUrl('create', sonata_admin.field_description.getOption('link_parameters', {})) }}"
            onclick="return start_field_dialog_form_add_{{ id }}(this);"
            class="btn btn-success btn-sm sonata-ba-action"
            title="{% trans from 'SonataAdminBundle' %}link_add{% endtrans %}"
            >
            <i class="fa fa-plus-circle"></i>
            {% trans from 'SonataAdminBundle' %}link_add{% endtrans %}
        </a>
    {% endif %}
    {% include 'SonataDoctrineORMAdminBundle:CRUD:edit_modal.html.twig' %}
    {% include 'SonataDoctrineORMAdminBundle:CRUD:edit_orm_many_association_script.html.twig' %}
    
    {# end copy / paste #}
    
    <script>
        (function ($) {
            var autocompleteInput = $('#{{ id }}_autocomplete_input');
            autocompleteInput.select2({
                {%- set allowClearPlaceholder = (not multiple and not required) ? ' ' : '' -%}
                placeholder: '{{ placeholder ?: allowClearPlaceholder }}', // allowClear needs placeholder to work properly
                allowClear: {{ required ? 'false' : 'true' }},
                enable: {{ disabled ? 'false' : 'true' }},
                readonly: {{ read_only ? 'true' : 'false' }},
                minimumInputLength: {{ minimum_input_length }},
                multiple: {{ multiple ? 'true' : 'false' }},
                width: '{{ width }}',
                dropdownAutoWidth: {{ dropdown_auto_width ? 'true' : 'false' }},
                containerCssClass: '{{ container_css_class ~ ' form-control' }}',
                dropdownCssClass: '{{ dropdown_css_class }}',
                ajax: {
                    url:  '{{ url ?: path(route.name, route.parameters|default([]))|e('js') }}',
                    dataType: 'json',
                    quietMillis: 100,
                    data: function (term, page) { // page is the one-based page number tracked by Select2
                        {% block sonata_type_model_autocomplete_ajax_request_parameters %}
                        return {
                                //search term
                                '{{ req_param_name_search }}': term,

                                // page size
                                '{{ req_param_name_items_per_page }}': {{ items_per_page }},

                                // page number
                                '{{ req_param_name_page_number }}': page,

                                // admin
                                {% if sonata_admin.admin is not null %}
                                    'uniqid': '{{ sonata_admin.admin.uniqid }}',
                                    'admin_code': '{{ sonata_admin.admin.code }}',
                                {% elseif admin_code %}
                                    'admin_code':  '{{ admin_code }}',
                                {% endif %}

                                {% if context == 'filter' %}
                                    'field':  '{{ full_name|replace({'filter[': '', '][value]': '', '__':'.'}) }}',
                                    '_context': 'filter'
                                {% else %}
                                    'field':  '{{ name }}'
                                {% endif %}

                                // other parameters
                                {% if req_params is not empty %},
                                    {%- for key, value in req_params -%}
                                        '{{- key|e('js') -}}': '{{- value|e('js') -}}'
                                        {%- if not loop.last -%}, {% endif -%}
                                    {%- endfor -%}
                                {% endif %}
                        };
                        {% endblock %}
                    },
                    results: function (data, page) {
                        // notice we return the value of more so Select2 knows if more results can be loaded
                        return {results: data.items, more: data.more};
                    }
                },
                formatResult: function (item) {
                    return {% block sonata_type_model_autocomplete_dropdown_item_format %}'<div class="{{ dropdown_item_css_class }}">'+item.label+'<\/div>'{% endblock %};// format of one dropdown item
                },
                formatSelection: function (item) {
                    return {% block sonata_type_model_autocomplete_selection_format %}item.label{% endblock %};// format selected item '<b>'+item.label+'</b>';
                },
                escapeMarkup: function (m) { return m; } // we do not want to escape markup since we are displaying html in results
            });

            autocompleteInput.on('change', function(e) {

                // console.log('change '+JSON.stringify({val:e.val, added:e.added, removed:e.removed}));

                // remove input
                if (undefined !== e.removed && null !== e.removed) {
                    var removedItems = e.removed;

                    {% if multiple %}
                        if(!$.isArray(removedItems)) {
                            removedItems = [removedItems];
                        }

                        var length = removedItems.length;
                        for (var i = 0; i < length; i++) {
                            el = removedItems[i];
                            $('#{{ id }}_hidden_inputs_wrap input:hidden[value="'+el.id+'"]').remove();
                        }
                    {%- else -%}
                        $('#{{ id }}_hidden_inputs_wrap input:hidden').val('');
                    {%- endif %}
                }

                // add new input
                var el = null;
                if (undefined !== e.added) {

                    var addedItems = e.added;

                    {% if multiple %}
                        if(!$.isArray(addedItems)) {
                            addedItems = [addedItems];
                        }

                        var length = addedItems.length;
                        for (var i = 0; i < length; i++) {
                            el = addedItems[i];
                            $('#{{ id }}_hidden_inputs_wrap').append('<input type="hidden" name="{{ full_name }}[]" value="'+el.id+'" />');
                        }
                    {%- else -%}
                        $('#{{ id }}_hidden_inputs_wrap input:hidden').val(addedItems.id);
                    {%- endif %}
                }
            });

            // Initialise the autocomplete
            var data = [];

            {%- if value is not empty -%}
                data = {%- if multiple -%}[ {%- endif -%}
                {%- for idx, val  in value if idx~'' != '_labels' -%}
                    {%- if not loop.first -%}, {% endif -%}
                    {id: '{{ val|e('js') }}', label:'{{ value['_labels'][idx]|e('js') }}'}
                {%- endfor -%}
                {%- if multiple -%} ] {%- endif -%};
            {% endif -%}

            if (undefined==data.length || 0<data.length) { // Leave placeholder if no data set
                autocompleteInput.select2('data', data);
            }

            // remove unneeded autocomplete text input before form submit
            $('#{{ id }}_autocomplete_input').closest('form').submit(function()
            {
                $('#{{ id }}_autocomplete_input').remove();
                return true;
            });
        })(jQuery);
    </script>
{% endspaceless %}