How to set the post excerpt when creating or editing a post

The af/form/editing/post_data filter lets you customise any post property — including post_excerpt — before Advanced Forms saves it. The filter applies on both create and edit.

The following snippet takes the value from a submitted field and assigns it to the new post’s excerpt:

add_filter( 'af/form/editing/post_data', function ( $post_data, $form, $args ) {

    // Only run for the target form.
    if ( $form['key'] !== 'YOUR_FORM_KEY_HERE' ) {
        return $post_data;
    }

    // Pull the value from your form field and assign it to post_excerpt.
    $post_data['post_excerpt'] = af_get_field( 'YOUR_FIELD_NAME_OR_KEY' );

    return $post_data;

}, 10, 3 );

The same pattern works for any other field on $post_data — post_title, post_content, post_status, post_author, etc.