Hi Ole,
Here is an example to customize the frontend submit button
add_filter( 'rwmb_frontend_submit_button', function( $submit_button, $post_id) {
$edit = 'true'; // or false, read the edit attribute here https://docs.metabox.io/extensions/mb-frontend-submission/#shortcode-attributes
$submit_button = '<button class="rwmb-button my-class" data-edit="' . $edit . '" name="rwmb_submit" value="1">Submit button text</button>';
return $submit_button;
}, 20, 2 );