The problem is that Iran can sponsor all the proxy aggression it wants and pay no consequences themselves. That has to change.

Okay - how?

I mean, it's easy to say that Iran's activities within a region within which it is a significant hegemonic power, has the largest army and is the second-largest country, and is in alliance with several other regional powers "has to change". It's like saying Russia has to stop invading Ukraine, or China has to stop menacing Taiwan. I mean, we'd all love it if Iran stopped acting like it was a dominant military power in the area...but they're not going to just because we want them to, or even because we threaten them.

Iran's a regionally powerful country, and one of the things that goes with being a regionally powerful country is that you can engage in proxy conflicts in the region. Because you've got too big a military footprint for anyone to really come after you in a way that could start an actual war.

How would Iran respond if Trump announced, "If the MV Behshad is still present in the Red sea when I become president, you will lose it."

They'd ignore it, knowing that the U.S. isn't going to start a hot war with Iran by sinking the Behshad. The Behshad lacks the capability to defend itself from an attack - literally every country in the region has the physical ability to sink it, if they wanted to. The reason that none of them have - even Israel - is because sinking the ship isn't worth the consequences that would flow from sinking it. It's no small thing to destroy a ship lawfully present in international waters, using your military to kill the citizens of another sovereign nation.