As a rough workaround that requires a certain level of trust of your
players: Create a "Voter" (or whatever you want to call it) character
and share it with all of your players, giving them full control. Voter will now appear in the "speaking as" dropdown list at the bottom of the chat window for each player. When you call for an anonymous vote, each player changes their "speaking as" to Voter
and either expresses their vote in the chat or whispers it to the GM.
If you get more votes than players, remind players to only vote once and
hold the vote again. Once you have the votes cast, remind players to switch their "speaking as" dropdown back to something other than Voter . Hope this helps!