You can have a banner of new site under the header of old site. Google doesn't care as long as you don't resort to link farming and some blackhat tactics. You can add rel=”sponsored” if you just want the traffic to new site instead of backlinks.
100% free :) so you get an idea I use the actual game to generate traffic for my niche, related to the game.
Nothing difficult or to do with SEO, just smart thinking.
Today $8.95, still about 15 hours to go until end of day:
So, cloakit has a index.php file AND javascript code.
I've copied the javascript code and placed it into the theme.liquid folder,within the <head>.
The index.php file is supposed to be uploaded or copied and pasted somewhere.
From my understanding, there isn't a clear place to upload that type...