functions.php

<?php

/**
 * Custom Smart Tags
 *
 * @author Bill Erickson
 * @see http://www.billerickson.net/code/wpforms-custom-smart-tags
 */
function ea_custom_smart_tags( $content, $tag ) {

    if( 'my_smart_tag' == $tag ) {
        $value = 'Testing 1 2 3';
        $content = str_replace( '{' . $tag . '}', $value, $content );
    }

    return $content;
}
add_filter( 'wpforms_smart_tag_process', 'ea_custom_smart_tags', 10, 2 );