Note: After saving, you have to bypass your browser's cache to see the changes. Google Chrome, Firefox, Microsoft Edge and Safari: Hold down the ⇧ Shift key and click the Reload toolbar button. For details and instructions about other browsers, see Wikipedia:Bypass your cache.
if(mw.config.get('wgAction') == 'edit') {
    addOnloadHook(function minorEdit() {
       document.getElementById('wpMinoredit').checked = true;
    });
}