It's really hard to say- US administrative law is really complicated. Could he make that threat? Sure, the President is fully empowered to speak in foreign affairs on behalf of the US government. He can threaten anything he wants. Could he carry it out without Congressional support? Maybe, he might be able to direct the Treasury by Executive Order to order the banks to do so as part of the Treasury's quite broad regulatory authority over financial institutions given to it by Congress, but it would be immediately challenged in court and I suspect ultimately President Trump would lose that challenge. Could he get Congress to support the threat with new legislation? I'd think probably not, but Congress has done a lot of things in the past few years I would have bet against.