what mod do i need to have a button to enter giveaways from the main page?

ESGST has this option. You can get it from here. Just follow the instructions on how to install the script, if it works you will see a new ESGST button on the top of the page on Steamgifts.

Click the button, type "giveaway button" in the search bar, and enable the setting in the list. This will put an Enter/Leave button for every giveaway on the front page.

A while back (before I even heard of this site, I was just browsing) I was looking through extensions on Firefox and found one called "AutoJoin For SteamGifts." I had no idea what that was, so I looked into it. Now I'm here at level 6 and steadily climbing. lol. Anyway, "AutoJoin For SteamGifts" has the "join" button on the front page, as well as several other nice features (my favorite is "infinite scroll" ... no more "next page", it just tacks onto the bottom of the current page when you scroll down that far). It does have the option to really do autojoin, but it flat out says don't do it because you can get your account banned ... not that I actually see much bot banning happening. Even if it was allowed, I wouldn't want to use the autojoin feature because there are some games (pretty much anything to do with sports, for example) I just don't ever want to touch. It also has the ability to display the description on the main page, but for me it doesn't show up until after I've already hit "Join" at which point it's kind of redundant. There are several other things it can do (like dark mode) that are cool but I don't bother with. I don't use ESGST (I tried it, it just didn't do anything that I felt I needed and I couldn't already do with Autojoin) but it seems most of the things Autojoin can do, it can as well ... so it's really a matter of personal opinion as to which one you're better using. I say try each one, turn ESGST off while trying Autojoin, and vice versa. See which one you like better.