Trigger Mobile Menu from a button

I am developing a site and have used shiftnav free to remove the menu and replace it with a “hamburger” menu at all times. Works great. However, the owner wants buttons at that bottom for contact, etc. Just a couple. One of them that he wants is a button that says “menu” and when it is clicked, it triggers the mobile menu at the top. I have been searching for hours trying to find an answer.

Is there a way to do this?

Thanks