Is it possible to trigger code in a signature field before it is actually signed?

As the title says, I want to know if it is possible to write a script in a Mouse Up action of a signature field and have that script execute before the field is actually signed.

Say I simply want an alert: 

app.alert("Signing now...");

So far any code I write happens only after the signature is completed. I would prefer not using an action like Mouse Enter or On Focus.

Any help is appreciated. Thanks.