Business process flow - run flow button validation

I want to run a flow from a Business Process Flow. 

However I have A text field which is JSON data for selecting multiple people. (Use a custom control to display people)

I want to perform a validation check on the field to make sure 1 or more persons are selected before clicking run flow. 

So In theory I want to know if there’s a OnClick event Handle I could grab from JS code maybe ? Or some other event I need to capture ? 

any ideas or work arounds to my issue ? 

thanks 

Jonathan