Understanding what spot on that form you fill in when you enter a hospital… I would say “yess, write in catholic” .
It’s the denomenation you connect with right now.
By filling in that spot, that puts you directly in line to recieve any spiritual support the hospital offers you: a catholic visitor, or a priest/deacon/religous.
If that’s who you want spiritually ministering to you at the time of being in the hospital, then by all means put down catholic.
God forbid, you were in immenant danger of death, a priest there at the time may be able to offer you sacraments.
God bless.
Just one note of caution: if you put catholic on your form…eucharistic ministers may come visit you with holy communion. Because you are not in full communion yet, you can not recieve communion. Although you could ask them to pray for you/ask for Spiritual commuion (where you pray to be forgiven anything that divides you from God , express your will to be in full communion with Him, offer him your whole self and ask that He comes to you spiritually and unites yourself to Him.