Does REDRIVERBANK participate in apple cash

Which question do you need an answer to? Your subject asks about a bank and Apple Cash - Ask your bank if they participate.

Your post asks about using a default account:

1. Open Settings and tap Mail.
2. Tap Default Account at the bottom.
3. Choose the desired email account to set it as the default.

Apple Cash is perhaps the safest money exchange system you can use. It is fully encrypted. I use ApplePay everyone I possibly can.