Name of form that a U.S. employer can fill to pay a Canadian employee?

Do you mean an independent contractor? It is difficult for a US company to directly hire a Canadian employee in Canada without either setting up a presence in Canada or using an EOR (employer of record which is like a presence in Canada as-a-service). This Canadian entity will pay the employee and make sure all Canadian taxes are withheld.

For an independent contractor you can avoid that hassle and pay them directly but the IRS might ask for proof that the contractor isn't a US citizen living in Canada (in which case you would have to send a 1099-NEC). So you should have them complete a form W-8BEN certifying that they are Canadian.

Note that I am not a tax lawyer and you should consult one before making any decisions.