Login_form add to top of form rather then bottom

Unfortunately there is no hook for above the fields (perhaps you might want to submit a patch/feature request on trac?)

You’ll have to get by with JavaScript:

<p id="klantnr-field">
    <label for="klantnr">Klantnummer<br>
    <input type="text" tabindex="20" size="10" value="" class="input" id="klantnr" name="klantnr"></label>
 </p>

<script>
    (function ( form ) {
        form.insertBefore( document.getElementById( "klantnr-field" ), form.childNodes[0] )
    })( document.getElementById( "loginform" ) );
</script>

And I hope you’ll be sanitizing that $klantnr variable if you plan on using it 😉