Add custom field when mint new ERC721 token

Question

i'm using the ECR721 preset smart contract from Openzeppelin for studying.

I would like to add a new field when i mint new token to store a string (public).

At the moment there is only the field "to:address" (screenshot below)

I'm sure i need to add something in the mint function:

Answer 1

Easiest way to store the message on blockchain is to emit an event. Event is permanently stored and publicly readable.

1. Define new MintMessage event outside the mint() function. I don't recommend expanding the default Transfer event that is used during minting, because external tools (such as Etherscan) might ignore the non-standard event and not show minted tokens as a result.
2. Add a new argument to the mint() function
3. Emit the MintMessage event within the mint() function

event MintMessage(string message);

function mint(address to, string message) public virtual {
   // keep the rest of your function as is
   // add a new line emiting the event to the end of the function
   emit MintMessage(message);
}