You can't. You can't write a pen name in the unimportant place, and the contract signed with a pen name is invalid. If you press your handprint, the contract is still valid although it was signed under a pseudonym. When one party signs a contract under a pseudonym, which causes the other party to express his true intention in favor of himself, it is a fraudulent act and an illegal civil act, which may lead to the invalidity of the contract from the beginning.