Hello @Drafter,
Thanks for writing to us.
It seems that you are using the X theme header, regretfully there is no option to add a different site URL on the login. I would suggest you upgrade your X theme to a Pro theme where you would get the option of a custom Header builder. You can create a custom Header through the Header builder where you can insert the custom URL on the logo.
Alternatively, in case you have an idea about the custom JS code, I would suggest you please add this code to global JS to get a custom link to the logo.
jQuery(document).ready(function($){
jQuery(".x-brand").attr("href", "https://www.youtube.com/");
});
Please feel free to change the Youtube URL to your URL. The purpose of providing the custom Javascript is to show you how to add Javascript code to your site. Writing custom is outside the scope of our theme support. If you need more customization, you need to learn Javascript https://www.w3schools.com/jquery/ In case if you have no idea about coding you can subscribe to One where customization questions are answered.
Hope it helps.
Thanks