flash.moe/public/userscripts/mpvfriend.user.js

// ==UserScript==
// @name     mpv friend
// @version  1
// @grant    none
// ==/UserScript==

/** mpvfriend.reg
Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\mpv]
@="URL:mpv"
"URL Protocol"=""
"DefaultIcon"="\"C:\\Users\\Julian\\Software\\mpv\\mpv.exe\",1"

[HKEY_CLASSES_ROOT\mpv\shell]

[HKEY_CLASSES_ROOT\mpv\shell\open]

[HKEY_CLASSES_ROOT\mpv\shell\open\command]
@="\"C:\\Users\\Julian\\Software\\mpv\\open.bat\" \"%1\""
**/

/** open.bat
@echo off
set URL=%1
mpv %URL:~0,1%%URL:~5,255%
**/

window.addEventListener('click', function(ev) {
    var target = ev.target;
    while(!(target instanceof HTMLAnchorElement)) {
        target = target.parentNode;
        if(!target)
            return;
    }
    if(!target.href)
        return;
    var url = new URL(target.href);
    if(url.protocol === 'mpv:')
        return;
  
  
    if((url.origin === 'https://www.youtube.com' && url.pathname === '/watch')) {
        ev.preventDefault();
        ev.stopPropagation();
        var open = document.createElement('a');
        open.href = 'https://flash.moe/mpv.php?url=' + encodeURIComponent(url.href);
        open.target = '_blank';
        document.body.appendChild(open);
        open.click();
        document.body.removeChild(open);
        return false;
    }
}, true);