Validate fields before signature

I have 2 fields in my form that I would like to make sure are not empty before people sign the form.

The names are:

Name candidate

NameEndClient

I have set these fields to Required, but it needs a trigger somewhere.

So I thought about running a JavaScript on the signature field (Signature1) with MouseUp, Action: Run A JavaScript

So when the Signature field gets clicked it would run the JavaScript and check if those two fields are not empty.

I have no idea where to go from here though

Can anyone help me here?