People will sometimes introduce themselves with pronouns, or sometimes wear little badges with them.

There are definitely people IRL who don't use the pronouns one might guess by looking at them. I haven't met many (any?) people who go by neopronouns, but they are around the Internet.

You can often just guess pronouns for people, but if you can't read the gender someone is presenting (is your new friend rocking a kilt, sports bra, and enormous beard?) it might be polite to ask, and/or to use "they" until you get the right one.

You don't have to want people to call you he/him just because you are a man. But he/him is overwhelmingly popular with men, so it's a fine choice.

If there is a field for pronouns, and you want people to know yours instead of them having to guess, you should put yours in there. The other reason to put pronouns in, even if people are likely to guess right, is to exercise the field for the people who often get guessed wrong.