Use a CMB2 options page to populate Ninja Forms dropdown: Use an options page to create dynamic select options for a Ninja Forms. Easier for a client to manage. https://www.damiencarbery.com/2019/01/use-a-cmb2-options-page-to-populate-ninja-forms-dropdown/

cmb2-options-page.php

<?php
/*
Plugin Name: CMB2 Options Page
Plugin URI: https://www.damiencarbery.com/2019/01/use-a-cmb2-options-page-to-populate-ninja-forms-dropdown/
Description: Use CMB2 to add an options page with repeater fields.
Author: Damien Carbery
Version: 0.2
*/

// Create an options page.
add_action( 'cmb2_admin_init', 'dcwd_register_course_dates_options' );
function dcwd_register_course_dates_options() {

	$cmb_options = new_cmb2_box( array(
		'id'           => 'hse_course_dates',
		'title'        => 'Course Dates',
		'object_types' => array( 'options-page' ),

		// The following parameters are specific to the options-page box
		'option_key'      => 'course_dates', // The option key and admin menu page slug.
		'icon_url'        => 'dashicons-calendar-alt', // Menu icon. Only applicable if 'parent_slug' is left empty.
		'capability'      => 'publish_posts', // Capability required to view options-page.
		'position'        => 2, // Menu position. Only applicable if 'parent_slug' is left empty.
		'save_button'     => 'Save Dates',
	) );
	
	$cmb_options->add_field( array(
		'desc' => 'Add dates for forthcoming courses.',
		'type' => 'title',
		'id'   => 'options_title'
	) );

	// Options fields IDs only need to be unique within this box. Prefix is not needed.
	$course1_dates_group_id = $cmb_options->add_field( array(
		'id' => 'course1_dates_group',
		'type' => 'group',
		'repeatable'  => true,  // This allows the group fields to be repeated.
		'options'     => array(
		'group_title'   => 'Course One {#}',
		'add_button'    => 'Add another Course One date',
		'remove_button' => 'Remove this Course One date',
		'closed'        => false,  // Repeater fields open by default so that the first repeater field get set.
		),	) );

	$cmb_options->add_group_field( $course1_dates_group_id, array(
		'name' => 'Course 1 date',
		'id'   => 'course_date',
		'type' => 'text_date_timestamp',
		'date_format' => 'j M Y',
		'attributes' => array(
			'required' => 'required',
		),
	) );
	$cmb_options->add_group_field( $course1_dates_group_id, array(
		'name' => 'Course 1 location',
		'id'   => 'course_location',
		'type' => 'text',
		'attributes' => array(
			'required' => 'required',
		),
	) );
}


// Verify that CMB2 plugin is active.
add_action( 'admin_notices', 'verify_cmb2_active' );
function verify_cmb2_active() {
	if ( ! defined( 'CMB2_LOADED' ) ) {
		$plugin_data = get_plugin_data( __FILE__ );
		$plugin_name = $plugin_data['Name'];
?>
<div class="notice notice-warning is-dismissible"><p>Plugin <strong><?php echo $plugin_name; ?></strong> requires <a href="https://wordpress.org/plugins/cmb2/">CMB2 plugin</a>.</p></div>
<?php
		//error_log( 'CMB2 is not active.' );
	}
}

ninja-forms-inject-select-options.php

<?php
// Inject the course info into the Ninja Forms Select options.
add_filter( 'ninja_forms_render_options', 'dcwd_add_course_dates_to_ninja_forms_select', 10, 2 );
function dcwd_add_course_dates_to_ninja_forms_select( $options, $settings ) {
	if ( 'listselect_course_date' == $settings[ 'key' ] ) {
		$dates = get_option( 'course_dates' );
		
		if ( $dates ) {
			if ( array_key_exists( 'course1_dates_group', $dates ) ) {
				$course_dates = array();
				$now = time();  // Used to exclude dates in the past.
				
				// Add the course dates and locations to an array and sort.
				// Only include courses for dates in the future.
				foreach ( $dates[ 'course1_dates_group' ] as $i ) {
					if ( $i[ 'course_date' ] >= $now ) {
						$course_dates[ $i[ 'course_date' ] ] = array_key_exists( 'course_location', $i ) ? $i[ 'course_location' ] : 'TBC';
					}
				}
				ksort( $course_dates, SORT_NUMERIC );  // Sort the courses by date.
				
				// Create the new options.
				$options = array();
				foreach ( $course_dates as $date => $location ) {
					$options[] = array(
						'label' => date( 'j F Y', $date ) . ' at ' . $location,
						'value' => esc_attr( date( 'j M Y', $date ) . ' at ' . $location ),
					);
				}
			}
		}
	}

	return $options;
}