Does the Constitution say if Trump declares himself the winner at any time on November 3 then he automatically wins? No, that's not true: This is satire from Houston talk radio host Michael Berry.
The claim was posted to Facebook and Twitter. A tweet (archived here) was published by Berry on November 2, 2020. It opened:
Constitution says if Trump declares himself the winner at any time tomorrow then he automatically wins. And he's gonna do it.
This is what the post looked like on Twitter at the time of writing:
(Source: Twitter screenshot taken on Tue Nov 3 14:21:57 2020 UTC)
While it is probably clear to the talk show host's audience that this comment was meant as satire to wind up liberals, some people who saw the post did not understand that context. There is nothing in the U.S. Constitution that says a candidate or incumbent president can simply declare themself the winner at any time during Election Day. The final results of the election may not be known for days or even weeks after the election.
The Official Guide to Government Information and Services offers this guide on the Presidential Election Process