All you need to do is search for “Pokemon vote” on Google. The voting screen will be the first thing you see among the search results. The interface is quite basic, so you’ll have to do quite some scrolling if your favourite comes far behind in their Pokedex number.
You’ll then be able to vote for one Pokemon from every region, representing every generation of the games to date. Since the vote uses the National Pokedex number, if your favourite critter has multiple forms, only their base forms count. While you can only vote for one Pokemon for each region each day, you can keep casting votes every day until 9:59PM of 14 February 2020. There will be other activities in conjunction with the upcoming Pokemon Day as well. One is Pokemon Go, which will have Pikachu and Eevee with party hats appearing beginning 25 February. For those who are playing Sword and Shield, there will be special Max Raids on the day itself. (Source: Pokemon)