While physically in the U.S., with very limited exceptions, a foreign national must have some form of U.S. work authorization to engage in revenue-generating activities, even if those activities are remote/virtual or occurring in another country. The default options, depending on the individual's country of citizenship (excluding those from Australia, Canada, Chile, Mexico, and Singapore, who have their own work visas), are the O-1, H-1B, and E-2.