Ninja Forms – Display dynamic drop down selection in Submissions - Dynamically add options to a 'Select' field and ensure that they show when editing a Submission. https://www.damiencarbery.com/2019/01/ninja-forms-display-dynamic-drop-down-selection-in-submissions/

nf-dynamic-select-options.php

<?php
/*
Plugin Name: Ninja Forms Select field - display in form and Submissions
Plugin URI: https://damiencarbery.com/
Description: Dynamically add options to a 'Select' field and ensure that they show when editing a Submission.
Author: Damien Carbery
Version: 0.1
*/

// Inspired by: http://developer.ninjaforms.com/codex/dynamic-list-fields/
// And from reading ninja-forms/includes/Abstracts/List.php and noticing that $_GET[ 'post' ] was used.


// Update 'Select' options when the form is being rendered.
add_filter( 'ninja_forms_render_options', 'dcwd_nf_forms_render_options', 10, 2 );
function dcwd_nf_forms_render_options( $options, $settings ) {
	if ( 'listselect_1546711953461' == $settings[ 'key' ] ) {
		$options[] = [
			'label' => 'New Seven',
			'value' => 'new',
			'calc' => 0,
			// If you want this value to be selected by default, only do this on the front end
			// as the selected value in admin (when viewing Submissions) should be the one that
			// was selected when the form was submitted.
			// If you don't want it selected on the front end then just set to false.
			'selected' => is_admin() ? false : true,
		];
		
		// If viewing a submission get the submitted value in case it is no longer an option in the form.
		if ( is_admin() && array_key_exists( 'post', $_GET ) ) {
			$post_id = absint( $_GET[ 'post' ] );
			$selected_value = get_post_meta( $post_id, '_field_' . $settings[ 'order' ], true );
			
			// Check whether the selected value is already in $options (either part of the form or added above).
			$key = array_search( $selected_value, array_column( $options, 'value' ) );
			// Only add the selected value if it's not present.
			if ( false === $key ) {
				$options[] = [
					'label' => $selected_value,  // The original display label is not available.
					'value' => $selected_value,
					'calc' => 0,
					'selected' => true,
				];
			}
		}
	}
	return $options;
}