We already prefill that key when pasting a delegation and its delegated key is recognized as an auxiliary key from this wallet.
Do it also when pasting a proof and its master pubkey is recognized.

Open the delegation editor in a wallet from which a key was used to produce a proof. Paste the proof in the "From proof" tab. Check that the delegator priv key field is prefilled.

Check also that the feature still works when pasting a delegation.