Mo. Rev. Stat. § 408.375: Retail installment agreement, deemed signed or accepted, when.

As used in sections 408.250 to 408.370, a retail installment agreement shall be deemed to be signed or accepted by the buyer, if after a request for a retail installment account, the agreement or application for a retail installment account is in fact signed by the buyer, or if the retail installment account is used by the buyer, or if the retail installment account is used by another person authorized by the buyer to use it.