I had Apple Pay with several bank and credit cards already set up. After installing 11.2 a few minutes ago, the phone rebooted and I went right into Wallet and at the top of my cards was the new Apple Pay Cash card. Tapped it to bring it to the front and hit the little “i” icon below it to get into the settings for the card and verified my account with SS# date of birth etc... then went back into cards setting in wallet to be sure I had the right card set as my default before then adding $20 to the card to test it.
Right as I submitted the deposit to the card, it alerted me right away that cash was added to the card.