Find the line:
<input type="text" name="ns1prefix" class="form-control" id="inputNs1prefix" value="{$server.ns1prefix}" placeholder="ns1">
Replace the line with:
<input type="text" name="ns1prefix" class="form-control" id="inputNs1prefix" value="ns1">
Find the line:
<input type="text" name="ns2prefix" class="form-control" id="inputNs2prefix" value="{$server.ns2prefix}" placeholder="ns2">
Replace the line with:
<input type="text" name="ns2prefix" class="form-control" id="inputNs2prefix" value="ns2">