How to cancel On Add Row event?

How to cancel On Add Row event?

Is there a "cancel Add Row" type syntax in the On Add Row subform event that can be fired?  My use case is this, I want the screen to send out an Alert saying that some fields are not filled out in the master/main form when the user clicks on the "Add" link in the subform and then remove the new row in the subform.

So in my On Add Row action, I have checked the condition say:

if (input.Field1 == null)
{ alert "Field 1 in the main form needs to be filled out.";
  cancel Add Row;  <===== Of course this line does not work.  

So is there an equivalent like cancel submission or cancel delete type syntax for Add Row here?

I have also tried "clear row;" and it didn't work neither.

Any help will be greatly appreciated.  Thanks.
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