Comment by foota

This may not mean what you think it means. From wikipedia:

"they were acting "at the order, request, or under the direction or control, of a foreign principal" and proved that the alleged foreign agent engaged "in political activities for or in the interests of such foreign principal," including by "represent[ing] the interests of such foreign principal before any agency or official of the Government of the United States."[31]"

You just have to be working for another government in a political way in the US to be a foreign agent.