Hi There,
May I suggest a different solution?
Why dont you use your form using the Converplug plugin?
This way you can activate it based on the class of the element.
S once you create your button and give it a class ex: modallauncher you can assign it as a launcher class on Convertplug to launch the modal.
For more information you can see this video. What you will learn does not need to be used necessary inside Cornerstone.
Let us know if you need further help.
Thanks,
Joao