wpmem_msg_dialog

// General used framework.
add_filter( 'wpmem_msg_dialog', function( $str, $tag ) {
    
    // filter the message based on the tag being passed.
    
    return $str;

}, 10, 2 );


// Specific example to change 
add_filter( 'wpmem_msg_dialog', function( $str, $tag ) {
    
    // filter the message based on the tag being passed.
    // "success" is the tag for successful registration.
    if ( 'success' == $tag ) {
        $str = "Thank you for registering. You may now sign in to view members only content.";
    }
    
    return $str;

}, 10, 2 );