StubHub gift codes are case-sensitive, which means you need to enter them exactly as they appear (including dashes).
Add a StubHub gift code on a desktop
Go to Profile > StubHub Wallet
Enter the gift code number
Add a StubHub gift code on the StubHub app
On the Review & Buy page at checkout, tap Gift codes
Tap the + sign
Enter the gift code number. Tap Add.
Check the code you want to use and Apply