Technically your trademark license can be oral, but it is never recommended. It is always much better to have a written agreement that is signed by both parties in case there are problems down the line. In order to enforce the laws governing topics like the licensee’s liability for paying to use the trademark or any unauthorized infringement, you need hard evidence of what the parties specifically agreed to. When both parties have different sides to the story, without a trademark license agreement form, it's much more challenging to prove the facts to enforce governing law.