Hidden Ninja Forms form field that captures PMPro membership level

ninja_forms_field_membership_level.php

<?php
/*
 * Add a hidden field in Ninja Forms to capture Membership Level (if user is logged in).
 *
 * For Ninja Forms version 3 and higher.
 */
// Create Membership Level field for Ninja Forms
add_filter( 'ninja_forms_register_fields', function( $fields ) {
	if ( class_exists( 'PMProMembershipLevelNFInput' ) ) {
		$fields['pmpromembershiplevel'] = new PMProMembershipLevelNFInput;
	}
	return $fields;
} );

// Add class for input field.
function my_pmpro_add_membership_level_class_for_nf() {
	if ( ! class_exists( 'NF_Abstracts_Input' ) ) {
		return;
	}
	
	class PMProMembershipLevelNFInput extends NF_Abstracts_Input {			
		protected $_name = 'pmpromembershiplevel';
		protected $_nicename = 'Membership Level';
		protected $_section = 'userinfo';
		protected $_icon = 'eye-slash';
		protected $_type = 'hidden';
		protected $_templates = 'hidden';
		protected $_wrap_template = 'wrap-no-label';
		protected $_settings_only = array(
			'key', 'label', 'admin_label'
		);
		public function __construct() {
			parent::__construct();
			$this->_nicename = __( 'Membership Level', 'ninja-forms' );
			$this->_settings[ 'label' ][ 'width' ] = 'full';
		}
	}
}
add_action( 'plugins_loaded', 'my_pmpro_add_membership_level_class_for_nf', 1 );

// Set value for pmpromembershiplevel field
function nf_default_value_membership_level( $default_value, $field_type, $field_settings ) {
	global $current_user;
	// Get the current user's membership level object.
	if ( function_exists( 'pmpro_getMembershipLevelForUser' ) ) {
		$level = pmpro_getMembershipLevelForUser( $current_user->ID );
	} else {
		$level = false;
	}
	// Get the current user's membership level name.
	if ( ! empty( $level ) ) {
		$current_membership_level_name = $level->name;
	} else {
		$current_membership_level_name = false;
	}
	if ( 'pmpromembershiplevel' == $field_type && in_array( 'pmpromembershiplevel', $field_settings ) && ! empty( $current_membership_level_name ) ) {
		$default_value = $current_membership_level_name;
	}
	return $default_value;
}
add_filter( 'ninja_forms_render_default_value', 'nf_default_value_membership_level' , 10 , 3);

This recipe is included in the blog post on "Ninja Forms: Add Hidden Field for Membership Level" at Paid Memberships Pro here: https://www.paidmembershipspro.com/ninja-forms-add-hidden-field-for-membership-level/