The term 'apply usage rights to form' is typically associated with an action involving LiveCycle Reader Extensions ES, through the server-side deployment of LiveCycle Reader Extensions or the client-side set of Reader Extensions services exposed in Acrobat Pro.
My answer does not apply to this use case.
Here's my suggestion:
1) add an email submit button called 'emailSubmitBtn' and set its presence to 'hidden'
2) add a button called 'emailBtn' and set the caption accordingly
3) on the 'emailBtn' click event set the target fields access to 'protected' and call the 'click' event on the email submit button 'emailSubmitBtn'
In the sample, below, I set the field 'name' to 'protected' and call the click event 'emailSubmitBtn'.
// form1.page1.emailBtn::click - (JavaScript, client)
form1.page1.subform1.name.access = "protected";
form1.page1.emailSubmitBtn.execEvent("click");
Steve