change
<input username="USERNAME" password="PASSWORD">
to
<input type='text' name='username' value="USERNAME" />
<input type="password" value="PASSWORD" name="password" />
and add a submit button:
<input type="Submit" value="Login" />
Then it'll work.
But, however if you don't have access to That sites login script, Then You're doing illegal thing called CSRF attack
(Cross Site Reference attack), And also a thing called Phishing
.
So please avoid it!!