you need a select<select> form element to create a dropdown list for the user to choose the recipient, then get the value of the selected option and change the action of the form to that by concatenating a string. based on the selected value.
document.forms[0].action = ("mailto:" + document.forms.selector.options[document.forms.selector.selectedIndex].value);
<select name="selector">
<option value="secretagent@cia.gov">secretagent@cia.gov</option>
</select>
I believe that should get the job done didn't check the code so there may be an error or 2
also you do realize that mailto does not actually send an email?