What fallacy informs the question I just heard on the radio, when the caller asked the host which other sitting president proclaimed fraud in his loss.

Clearly, the use of this technique shows bias and might even be deceptive. However, sometimes creating "gotcha" questions are legitimate uses of the Socratic method, that help people to come to conclusions on their own. When using Soratic reasoning, the one asking the questions already knows the answer. The point is, "gotcha" questions aren't the real problem.

There is an implied argument here, and you correctly addressed that. We can reasonable infer that the argument is "no other sitting president claimed fraud in his loss; therefore, Trump is [a bad guy, sore loser, whatever]". We should get the person to state this argument directly, then address the argument. Perhaps they are being purposely ambiguous so they don't have to make an argument and defend it. Perhaps they are trying to poisoning the well by creating a narrative that such challenges are unheard of and therefore wrong, which is a non sequitur .

I wouldn't call an initial fallacy, but insist they state their argument, then look for fallacies.

To me, the wording shows a bias on the part of the questioner as distinct from a genuine intellectual interest in history. 

For example, the question assumes a "fact not in evidence:" that Trump lost. As of this writing (November 28, 2020), who will become president is undecided. There is no "president-elect."

Trump has insisted that if only legal votes are counted, he won. Perhaps he is right. Hopefully we will learn. For example, the Georgia 104-page lawsuit lists some thirty instances of serious fraud. One might think of Trump’s position as demanding a transparent election to make sure Stalin's ghost is not raised here. “It’s not the people who vote that counts. It’s the people who count the votes.” – Josef Stalin.

Also, due to the publicized irregularities, there are other avenues to the office beside the popular vote.

Perhaps an "Alleged Certainty" fallacy. The question alleges (or implies) that there was a past president who proclaimed fraud, without a plausible reason to believe that to be so.