How to set up an in-browser proxy for only selected websites?

[SurpriZe]

I've been using this Opera 12 feature of a proxy connection being applied to only a selected website list for a very long time now, and the fact that I cannot set it up in Vivaldi bothers me, since I have to resort to using unreliable extensions, making the experience so much worse.

The point is that I input a certain proxy into my "proxy settings" and it will work only for a few selected websites I added myself, while everything else works through the standard connection.

Does anybody have any suggestion on how I can set it up in Vivaldi? The current settings allow for only the reverse solution, where I can add the websites for which NOT to use the proxy, but I need the opposite.