JSFiddle - React, Tailwind, and code Playground

HTML

<select name="country" id="country" onchange="showhide('1', '1')">
    <option>test</option>
    <option>test2</option>
</select>

<select name="region_name1" id="region_name1" disabled="disabled">
    <option>test</option>
    <option>test2</option>
</select>

JavaScript

function showhide(id,num){
    if (id == '1' || id == '100500'){
    document.getElementById('region_name'+num).disabled = false;
    }
}