You can use the following Dynamic SQL to build the query you need to hit all databases and get a list of Users:
DECLARE @LoginName AS NVARCHAR(100) = N'Your Login Name Here'
DECLARE @DynamicSQL AS NVARCHAR(MAX) = N'';
SELECT @DynamicSQL = @DynamicSQL +
'SELECT ''' + [name] + ''' AS DatabaseName, [name] AS UserName FROM [' + [name] + '].sys.database_principals WHERE [type] IN (''S'', ''U'', ''E'', ''G'') AND [name] = ''' + @LoginName + ''' UNION ALL '
WHERE [state] = 0; -- Online databases only
SET @DynamicSQL = LEFT(@DynamicSQL, LEN(@DynamicSQL) - 10); -- Cleans up unneeded trailing "UNION ALL"
-- PRINT @DynamicSQL -- Prints the dynamic SQL string (For Testing)
EXEC master.sys.sp_ExecuteSQL @DynamicSQL -- Executes the dynamic SQL string
I based it off the query Aaron Bertrand is using in the answer that @Learning_DBAdmin linked, but I figured re-writing my own Dynamic SQL could make it a little more readable. (And as he mentions sp_msforeachdb is undocumented and unreliable, so better to query dynamically.)
This can be used to get SQL Users, Windows Users, Azure Active Directory External Users, and Windows Groups based on the Type column in sys.database_principals (you can adjust this as needed).