Doing this fixed my issue
<script type="text/javascript">
function SubmitFrm(){
var Searchtxt = document.getElementById("txtSearch").value;
window.location = "http://www.mysite.com/search/?Query=" + Searchtxt;
}
</script>
I changed .value();
to .value;
taking out the ()
I did not change anything in my text field or submit button
<input name="txtSearch" type="text" id="txtSearch" class="field" />
<input type="submit" name="btnSearch" value="" id="btnSearch" class="btn" onclick="javascript:SubmitFrm()" />
Works like a charm.