MS SQL, invalid object name

Okay, let me make it short, I can delete a whole row from database using this code:

$sql = "DELETE FROM [master].[dbo].[TestTBL] WHERE Agent_ID = '{$_SESSION["agentid"]}' ";

but when I try:

$sql = "DELETE username FROM [master].[dbo].[TestTBL] WHERE Agent_ID = '{$_SESSION["agentid"]}' ";

I also tried to make it [name], but I still get Invalid Object Name username...

I want to delete only the username of a certain Agent_ID Any help will be much appreciated. :D