Trigger the right event when a form opens on a page

Trigger the right event when a form opens on a page

My problem is that I don't know what is the right event when a form opens on a page.
Specifically, my webpage is as follows:
When the visitor clicks the upper button (with the envelop icon) it launches a form.
I need to set  a specific value to one of the fields with jQuery at that moment.
Is it possible and how?
Thx