If your iPad has a Lightning connector, it will work with your YubiKey. If your iPad has a USB-C connector, it will not work with your YubiKey.
Strongbox, like all KeePass based apps, uses a message authentication code (MAC) called HMACSHA1. Unfortunately it is currently not possible to use HMACSHA1 over USB-C on iPad. Also, iPads do not support NFC.
Please note, this is because of design decisions made by both Apple and Yubico. We encourage USB-C iPad users to contact Apple and Yubico directly and request that they add full support for USB-C!
For the list of compatible YubiKey keys please see Yubico's Strongbox product page here: