question

You can still use GetAccountInfo method to find a user with PlayFab Username instead of PlayFabId. When you want to search with user name, you will need to set Username property in GetAccountInfoRequest instead of PlayFabId. For example:

PlayFabClientAPI.GetAccountInfo(new GetAccountInfoRequest { Username = "theuser" },
    result =>
    {
        //Handle AccountInfo
        Debug.Log(result.AccountInfo.PlayFabId);
    },
    error => { Debug.LogError(error.GenerateErrorReport()); });

And the Username is the one that showed in Master player account as PlayFab username.

However, please note searching on a user is on a match only - it's not a wildcard match, as that could return thousands, or even millions, of records. So if you set Usernameto "the", you will not get the AccountInfo of "theuser", but a "AccountNotFound" error.

Besides, I'm not sure what do you mean "it was said to be too long"? The PlayFabId would be a string with 16 characters, if you are using a valid PlayFabId, it should not be too long.