add_filter( 'streamtube/core/video/download_file_url', function( $url ){

    // define your custom sub domain
    $sub_domain = 'https://mysub.domain.com';

    $attachment_id = get_post_meta( get_the_ID(), 'video_url', true );

    if( ! wp_attachment_is( 'video', $attachment_id ) ){
        return $url;
    }

    $file_url = wp_get_attachment_url( $attachment_id );

    return str_replace( untrailingslashit( home_url() ), untrailingslashit( $sub_domain ), $file_url );

}, 100, 1 );