Set FormInput Default Value from HTML Select

Question

This might be a bit of a reach as I know very little about PHP, JS and HTML. I am in the process of making a new form, which on submit will create a new incident within OSC. The form requires the use of a dynamic drop down list, which I created using JavaScrpit and HTML Select.

I need to find a way to get the value from this second Select element into OSC when the user presses submit. The thing is I don't know how to refresh the widget each time the dropdown is changed in order to pick up the default value.

Any help would be appreciated and my attempt is below:

<p>File Type</P>            
        <!-- start custom drop down dependency elements -->
        <!-- Level 1 Selector -->
        <select id="slct1" name="level1" onchange="dform(slct1,slct2)">
            <option value="">Please Select</option>
            <option value="Absence">Absence</option>
            <option value="Benefits">Benefits</option>
            <option value="Check">Check</option>
        </Select>
<!-- Level 2 Selector controlled via JS -->
        <select id="slct2" name="level2"></select>

        <!--<div style="display:none"> -->
        <?php $doctype=$_GET["level2"];
        ?>
        <rn:widget path="input/FormInput" name="incident.c$doc_type_upload" required="true" default_value=#rn:php:$doctype#/>