<option value="subject_only"', ($selected == 'search_selection' ? '' : ''), '>','Topic Titles', '</option>
<input type="hidden" name="subject_only" value="0" />
if it's 0, the search area ignores it
JavaScript to add at the end off theme.js
window.addEventListener("DOMContentLoaded", function()
{
document.forms.search_form.addEventListener('submit', function()
{
let
el = this.search_selection,
val = el.options[el.selectedIndex].value;
this.subject_only.value = val !== 'subject_only' ? 0 : 1;
});
});