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.
We (Canada) literally signed the United States-Mexico-Canada Agreement for trade in 2020... with Donald Trump. Trump this February said "Who would ever sign a thing like this?" and proceeded to make up the story about fentanyl coming from Canada as a pretense to ignore this deal. So there was certainly more than a pinky promise.
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.
> 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.
That bill won't pass the crazy house, but yes, CUSMA was signed law.