Hello Tanja,
You can use the filter hook rwmb_frontend_welcome_message
to adjust the welcome message of the frontend dashboard. Here is an example:
add_filter( 'rwmb_frontend_welcome_message', function( $message, $user, $query ) {
$message = '<h3>' . esc_html( sprintf( __( 'Howdy, %s!', 'mb-frontend-submission' ), $user->display_name ) ) . '</h3>';
$message .= '<p>' . esc_html( sprintf( __( 'You have %1$d posts.', 'mb-frontend-submission' ), $query->post_count ) ) . '</p>';
return $message;
}, 10, 3 );
Refer to the documentation https://docs.metabox.io/extensions/mb-frontend-submission/#dashboard