Copy the rpc_endpoint information from the Subscription details on the My NaaS tab on the App

Open Metamask and click on Network selector at top (you are probably defaulted to Ethereum mainnet).

Click on Custom RPC in that menu. That will open the "New Network" screen as shown on this page.

Enter a Network Name you want to call this network.

Paste the rpc_endpoint information into New RPC URL field.

Set Chain ID to 1.

Symbol and Block Explorer URL are optional. Leave them blank if you prefer.

Click Save.

You will now have Metamask connected to the custom RPC Endpoint.