Q: how can my banking institution / credit union request to have their issued debit/credit cards to used as Apple Pay payment cards?

Use the Contact Us links at the bottom of each page here. They would need to call or write.

Hi

Apple has not published a contact route for card issuer enquiries.

However, in the first instance, your credit union can contact the Apple Pay Merchant Support team via the feedback form:

https://www.apple.com/support/apple-pay/feedback/