Iowa

"The binding presidential vote functions essentially like a party-run primary, only with very limited polling hours and no accommodation for absentee voting, except for a tiny handful of overseas and military voters. There are speeches on behalf of various candidates before the voting and a variety of party business after the vote. Individual caucus chairs are allowed to exercise some discretion in how to conduct the vote, but the voting is done by secret ballot and there is no set list of candidates. Voters must be given the option to vote for any candidate they choose. In the past, some caucus sites have pre-printed the names of major candidates and provided a write-in option, but typically, voters vote by writing the name of a candidate on a blank slip of paper."

Here's a good primer:

https://apnews.com/article/how-iowa-caucus-works-2024-democrats-republicans-592ab40b9b9b948c0540f2cf132bab5c