Follow these steps:
Sign in to your Stage Meta Account.
Open your 'My Wallet' section.
Go to the 'Private Key' section.
Click on 'Export Private Key'.
Confirm the operation with the code sent to your email inbox.
After the confirmation, your alphanumeric Private Key will show, you can copy it from this window.
Open the MetaMask wallet.
Open the section to Import Wallet.
Follow the steps to Import your TPA Wallet using a Private Key.
Check this article for details on how to import your account to MetaMask.