Can I use my Beehaw account to log in to feddit.online? Or I have to make a new account?

So you can't use the front page of feddit.online through beehaw, you will need an feddit.online account for that. However, the same posts on feddit.online (most I see on the front page are from beehaw, Lemmy communities or kbin.social), you can participate from beehaw. For example https://feddit.online/m/kbinStyles@kbin.social is roughly equivalent to https://beehaw.org/c/kBinStyles@kbin.social

You'd have to make a new account but why though? You ought to be able to subscribe to their communities with thus account. :)

The best way to interact with communities on remote instances is to first subscribe to them, so you can be sure your instance receives all content. The way to do this is to copy the community URL and pasting it in the search box of your instance. That should fetch it (if it wasn't already) and then you can subscribe.