The private keys from your cold-storage?
1. Create the transaction on the watch only;
2. Save the raw txt and send it to the cold storage offline wallet (e.g with a QR code or a txt file in a flash drive);
3. Import and sign the transaction;
4. Save the signed transaction, send it back to the watch only address and broadcast it (or broadcast it directly from a website like
https://www.blockchain.com/pt/btc/pushtx)
This video aso shows exactly how to do it:
https://youtu.be/DQumISxkJsQ
Or maybe you have a private key (not related to your wallet) and want to send the coins on it to your wallet?
In the watch-only address, go to Wallet -> Private keys -> Sweep; paste your private-keys and the coins will be swept to your wallet.
