make a link that autofill an input in the linked page

[Scanu]

Ok this works like a charm but when i try to put this code on vbulletin i've some problems
Page A is in the header template and i have this code

<form method="post" action="register.php">
<input type="text" name="email"><input type="submit" value="Continue"></form>

When i click continue it says

Your submission could not be processed because a security token was missing.

If this occurred unexpectedly, please inform the administrator and describe the action you performed before you received this error.

What's the problem?