WP Plugin Specific Form Processing ('POST' via admin-post.php)

WP 4.9.1 bbPress 2.5.14 Plugin: GD bbPress Toolbox Pro 5.0.4 (GDT) 1) GDT offers a signature field for users (shown in bbPress). 2) This signature field can be implemented outside the plugin into child theme templates via PHP 3) I managed to get that field displayed via a specific php template but I can't save the data when updating the signature content because I don't understand how this function offered by GDT works. 4) I assume the easiest way is to rely on WP's core [login to view URL] and to call/place the correct "function" in my child theme's function.php. Please see attached screenshot with a brief description how to a) insert the respective "Signature Field" (I managed to do that) b) the respective form POST function (which I struggle with correct implementation) If you are versed with generic function<>[login to view URL] and/or GD bbPress Toolbox ([login to view URL]) please contact me with a suggestion how to process. Thanks!