This approach will only work from apps in the same apple developer account :/
nostr:npub1yaul8k059377u9lsu67de7y637w4jtgeuwcmh5n7788l6xnlnrgs3tvjmf is working on an iOS signer so you don't have to anymore, but until then there are no options except to paste in your nsec and hope for the best.
On some web apps you have the option to use nsec.app for remote signing, but the UX is pretty rough...
Discussion
That's unfortunate... I had not heard that. iOS really can't have nice things...