You can notarize online with HelloPrenup even if you are outside of the US with Proof as long as you have a US or foreign-based residence you can input and verify with Proof.
Additionally, if you are in the US, you may go to a local in-person notary, such as at a UPS to have your prenup notarized in person, if you do not want to use Proof.
However, note that you should not go to an in-person notary outside of the US, as it may not qualify for the same standards of notarization as is in the US.
Why does this matter?
Notarization helps deter arguments for fraud around signing a prenup (if your future spouse ever challenged your prenup, having a prenup notarized helps your argument)
Some states require notarization as a mandatory step, and without it, the prenup is automatically invalid.
Make sure this is done before the wedding (prenups signed and notarized AFTER the wedding are invalid).