fluent_form_dynamic_options.php

<?php

add_filter('fluenform_rendering_field_data_select', function ($data, $form) {
    if ($form->id != 91) {
        return $data;
    }
    // do the dynamic part if and only the name attriibute is 'dynamic_dropdown'
    // Please use the corresponding field name for your case. 
    if (\FluentForm\Framework\Helpers\ArrayHelper::get($data, 'attributes.name') != 'dynamic_dropdown') {
        return $data;
    }
    
    // We are merging with existing options here
    $data['settings']['advanced_options'] = array_merge($data['settings']['advanced_options'], [
        [
            "label"      => "Dynamic Option 1",
            "value"      => "Dynamic Option 1",
            "calc_value" => ""
        ],
        [
            "label"      => "Dynamic Option 2",
            "value"      => "Dynamic Option 2",
            "calc_value" => ""
        ]
    ]);
    return $data;
}, 10, 2);