Pre-populate Ninja Forms fields from URL parameters

functions.php

<?php

add_filter( 'ninja_forms_field', 'wpm_handle_http_query_string' );
function wpm_handle_http_query_string( $data ) {
	
	// A list field with label "Tier"
	if($data['label']=='Tier' && get_query_var('tier')) {
		foreach ($data['list']['options'] as $key => $value) {
			if($value['value'] == get_query_var('tier')) {
				$data['list']['options'][$key]['selected'] = 1;
			}
		}
	}
	
	// A textbox field with label "First name"
	if($data['label']=='First name' && get_query_var('first-name')) {
		$data['default_value'] = get_query_var('first-name');
	}

	return $data;
	
}