Adding an NFT to Metamask manually is very easy. You just have to find 2 informations related to your NFT. For instance, this is the process that has to be done when you received an Airdrop (free NFTs). As Metamask don’t get the information, you just have to do it by yourself to display your Free NFT on your Metamask personal NFT collection.
Find the Contract Address and the Token ID of your NFT
When you receive an Airdrop from Metav.rs, once your NFT is minted, we just display the Contract Address of the Collection and the ID of the Token. Just Copy them into Metamask.
Copy and paste it to your Metamask
Make sure to select the correct mainnet, usually Airdrop are on the Polygon Blockchain, click on Import an NFT and paste the values.