Using script in a dummy submit button to check for digital signature

Is it possible to add script to a dummy submit button to stop it from submitting if a signature field is not signed?  Perhaps adding something like what I have below.  I know the syntax isn't right.  I just haven't been able to get it to work.  Thanks.

if (SignatureField1 == null)

     {

     xfa.host.messageBox("Please digitally sign before clicking Submit!");

{