JSFiddle - React, Tailwind, and code Playground

HTML

<form name="searchform" onSubmit="return dosearch();">
  Search:
  <select name="sengines">
    <option value="option_0" disabled selected>Select an option by category</option>
    <option value="option_1">Option 1</option>
    <option value="option_2">Option 2</option>
    <option value="option_3">Option 3</option>
    <option value="option_4">Option 4</option>
  </select>
  For:
  <input type="text" name="searchterms">
  <input type="submit" name="SearchSubmit" value="Search">
</form>

JavaScript

function dosearch() {
   const sf = document.searchform;
   const selected_option = sf.sengines.options[sf.sengines.selectedIndex].value;
   if (selected_option === "option_0") {
     alert('Choose an option from dropdown first.')
   } else {
     const submitto = sf.sengines.options[sf.sengines.selectedIndex].value + escape(sf.searchterms.value);
     window.location.href = submitto;
     return false;
   }
 }