Add a hidden field to capture Membership Level on a Ninja Form. For Ninja Forms older than version 3.

register_ninja_form_fields.php

<?php
/*
 * Add a hidden field in Ninja Forms to capture Membership Level (if user is logged in).
 *
 * For Ninja Forms older than version 3
 */
function pmpro_register_ninja_form_level_field() {
	$argsMl = array(
		'name' => 'Membership Level',
		'display_function' => 'collect_membership_level_display',
		'sidebar' => 'template_fields',
		'display_label' => false,
		'display_wrap' => false,
	);

	if ( function_exists( 'ninja_forms_register_field' ) ) {
		ninja_forms_register_field( 'user_membership_level', $argsMl );
	}
}
add_action( 'init', 'pmpro_register_ninja_form_level_field' );

function collect_membership_level_display( $field_id, $data ) {
	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;
	}

	// Set the hidden level name field in the form.
	if ( ! empty( $current_membership_level_name ) ) { ?>
		<input type="hidden" name="ninja_forms_field_<?php echo $field_id;?>" value="<?php echo $current_membership_level_name; ?>">
		<?php 
	}
}

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/