You can apply for a BetMGM Play+ account by choosing Play+ from among the BetMGM payment methods in the cashier. Once there, fill in the requested details, and when you're approved you'll be able to start using your Play+ card instanly.
Within a few days, a physical version of the BetMGM Play+ card will arrive in the mail. You can use that card to withdraw cash from an ATM, or use it for purchases wherever Discover credit and debit cards are accepted.