I think you've got jQuery up and running? If so, you can do:
HTML code:
<input type="text" id="code" value="AT"/>
<button type="button" onclick="location.href='./fullc.php?code='+$j('#code').val();">Go To</button>
If not, it's slightly longer but not too bad:
HTML code:
<input type="text" id="code" value="AT"/>
<button type="button" onclick="location.href='./fullc.php?code='+document.getElementById('code').value;">Go To</button>