Verbal contracts can be as valid as written ones, including verbal contracts made for loans.
A promissory note (or any contract) created after the fact need only be signed if the written terms match the agreed upon verbal terms, and if both parties agree to sign the document created.
No one can ‘force’ another person to agree to and/or sign a legal document, by the very definition of contracts.
To easily create a valid note that is set up to be fair to both sides of a loan agreement, see Standard Legal’s Promissory Note legal forms software.