For example You can achieve this with jQuery function on dropdown value change event and setting element href attribute.
If You want, change <select /> to @Html.DropDownListFor() or @Html.DropDownList()
<script type="text/javascript">
$(document).ready(function () {
$('.myClass').change(function () {
$('.mbtn').attr('href', '/MyController/CityInfo?cityid=' + $(this).val());
});
})
</script>
<select class="myClass">
<option value="1">Texas</option>
<option value="2">Sao Paulo</option>
<option value="3">Mexico</option>
</select>
@Html.ActionLink("More Info", "CityInfo", null, new { @class = "mbtn" })
For more parameters You can for every dropdown set class='myClass' and different id. With it You should be able to use it as:
<script type="text/javascript">
$(document).ready(function () {
$('.myClass').change(function() {
$('.mbtn').attr('href', '/MyController/CityInfo?cityid=' + $('#cityId').val() + '&userId=' + $('#userId').val());
});
})
</script>