title: onClick event for Google Forms Submit button
body: <p>This is driving me crazy, I have a simple Google Form that accepts subscriber code and their slot timings. My purpose is to get data via Google Forms and save it to Google Sheets <strong>without having any clashes in the slots timings</strong>.</p>
  <p>To show you an example, I am attaching an image of my form and sheet. After the first entry was done, if another slot is to be given to same subscriber, we need to ensure that the slot timings do not clash with the saved timings in google sheet that is assigned to the same subscriber. If you see the second entry, it just accepts it and for this we need a validation check which is not available. My question is, is it even possible to have an onClick validation on the submit button? I have been searching for it on google like crazy but haven't got a solution. If this is not possible to do it in Google forms, what are my alternatives or is there any workaround available?</p>
  <p>Eagerly awaiting responses for my query. Thank you.</p>
  <p><a href="https://i.stack.imgur.com/RC6GM.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/RC6GM.png" alt="enter image description here" /></a></p>