Greasemokey - submit login form. Why are only 2 out of 3 values ​​sent?

Question

Greasemonkey

I'm trying to write a Greasemonkey script for autologin on rutracker.ru. Here is the form:

<form id="top-login-form" action="http://login.rutracker.org/forum/login.php" method="post" style="display: inline;">
<input id="top-login-uname" type="text" placeholder="имя" accesskey="l" tabindex="1" name="login_username">
<input id="top-login-pwd" type="password" placeholder="пароль" tabindex="2" name="login_password">
<input id="top-login-btn" type="submit" tabindex="3" value="вход" name="login">
</form>

And here is the script:

var f = document.getElementById("top-login-form");
f.elements.namedItem("login_username").value = "xxxxxx";
f.elements.namedItem("login_password").value = "yyyyyyy";
f.elements.namedItem("login").value = "%E2%F5%EE%E4";
f.submit();

When logging in manually, the Post request looks like this:
login_username=xxxxxxx&login_password=yyyyyyy&login=%E2%F5%EE%E4
When logging in using the script, only two of the three fields are sent, that is, the Post request looks like:
login_username=xxxxxxx&login_password=yyyyyyy
Please tell me why the login field is not sent, or with what tools can you try to track down what the problem is?