What should I do if my advisor is forcing me to vote on a specific candidate in IEEE elections?

Allow me to hypothesize a different tack from most of the other answers: Simply don't engage in this issue at all. As my mentor said once, "You constantly get institutional demands for some action, in some cases it's best to ignore them, and you'll find that many just go away."

This request is so incredibly unethical, and also so completely orthogonal to your responsibilities as a graduate student, that it's hard for me to imagine your advisor spending time on tracking you down and taking revenge over it. If you think you might receive more pressure later on (e.g., I've sadly cracked under such circumstances), go ahead and cast the online vote in the privacy of your own home, and then later on say, "Oh sorry, I got a notification and cast the vote immediately."

The request is so inflammatory that I don't see any use to "politely having an open discussion with your advisor". I think that only opens the door for defensiveness and retaliation. As wildly unacceptable as it is, I don't see it as being a good use of your time tracking down where to lodge a complaint over this matter (which your advisor seems canny enough to avoid documenting except verbally, so it would be just your word against his, according to comments above).

At least consider the null action as a possibility.

This is extremely unethical. Do you have any proof of what he asked for ? If he can't check for who you did vote, there is no problem for you (but the moral problem is still here), you might just say you did it alone, without the Assistant Professor. (You do it, then that's it.)

The ethical way would be to inform your university. But that is really dangerous. You might want to wait until the end of your graduate program. It is hard to find a good way to solve this problem without harming you.

In your position, I would certainly tell the teacher that I don't want to do it without a good reason and the certitude that is a good choice, but if you're not confident you can't handle the consequences, don't do it.

It would be even more powerful to inform the IEEE (if the OP can muster the proof that this coercion is going on). But, unfair as it is, it will probably damage the OP's career irreparably. Utterly despicable - I heard such stuff only from hearsay (or from infamous examples in 20th century history). Collect evidence if you can, and, at some point in the future you may be in the position to dispense justice.