Whatever... :)
The easiestway to do this will be as follows: copy everything between tags <form> and </form> in original login page and paste it into the external page at your site. That part of code looks as follows in my case (table tags deleted, cyrillic stuff changed to english):
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<form method="post" name="frmLogin" action="login_user.asp?FID=0" onSubmit="return CheckForm();" onReset="return confirm('Form reset');">
User Name: <input type="text" name="name" size="15" maxlength="15" value="" />
password: <input type="password" name="password" size="15" maxlength="15" />
<input type="checkbox" name="AutoLogin" value="True" checked />Auto Login
<input type="checkbox" name="ActiveUsers" value="True" checked />Active users
<input type="hidden" name="sessionID" value="538760709" />
<input type="submit" name="Submit" value="Login" />
</form>
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Don't forget to mention the full URL (just in case :) to actual forum login page, and replace SessionID with the actual one for each user.
Good luck