It's already there in the Constitution - Article II, Section 2, Clause 2. Unfortunately other countries chose not to require things in writing and were happy with pinky promises that could be reneged.
Additionally your senate just signed law to repeal the tariffs against Canada specifically so I don't know what other receipts or paperwork you'd like here. Not that I expect Trump to adhere to that of course, but I've never heard the perspective of "well these weren't signed agreements" for things that clearly were.
That bill won't pass the crazy house, but yes, CUSMA was signed law.