how can i enter multiple games in the 'have' field of steamtrades?

Did you try just adding a space between each entry?

I have no idea what you mean

Edit,
nvm, youre asking about search field in trades section.

Sadlty i did not find any separation character to work as logical [OR], its alaways [AND]

I guess you just have to try searching multiple times if/until there's a match.

AFAIK, search fields on this site can only interpret search queries as phrase searches, so it tries to find the exact long string you type there. Meaning, you can search for one game only, and even then it is better to use only one word from the title in case somebody displayed it incorrectly (like as, for example, No Mans Sky, as I see it quite often).

search one game at a time. this site doesn't have filters or special parameters to look for stuff.

The search function is pretty rudimentary, since all it looks for is a matching string so: "race car" and "car race" give different results.
So, there's no such thing available at this time.

You might try barter.vg, You can enter your Haves and your Wants and it will generate a list of all matches.

While the trading section here is nice, searching just isn't very robust.

it's 2018. We still don't have this

I made this tool. please check!!
https://www.steamgifts.com/discussion/PnjQN/userscript-steamtrades-show-matching-game-in-each-trade-with-your-havewant-list