To escape a characther in the LIKE clause of SQL, use the additional ESCAPE clause which is part of the SQL standard and should work with any relational DB.
WHERE username LIKE '%^_d' ESCAPE '^';
In the example above, the ^ is used as the escape charachter, which forces the query return all usernames that end with _d
Sepcifically in Microsoft T-SQL we can use also the character range operator: [], so the query becomes:
WHERE username LIKE '%[_]d'