WPForms register/create new smart tag

functions.php

<?php
/**
 * Registers the Request URI smart tag.
 *
 * @param array $tags
 * @return array
 */
function wpf_smarttag_request_uri_register( $tags ) {

	$tags['request_uri'] = 'Request URI';

	return $tags;
}
add_filter( 'wpforms_smart_tags', 'wpf_smarttag_request_uri_register' );

/**
 * Processes the Request URI smart tag.
 *
 * @param string
 * @param array $tags
 * @return string
 */
function wp_smarttag_request_uri_process( $content, $tag ) {

	$request = esc_url( remove_query_arg( 'placeholderarg' ) );

	return str_replace( '{request_uri}', $request, $content );

}
add_filter( 'wpforms_smart_tag_process', 'wp_smarttag_request_uri_process', 10, 2 );