You need to add this class custom-menu-primary in your main header class and paste the below css and script either in that custom module or in respective css and javascript then your toggle option will come in mobile menu
https://github.com/bradhave94/HubSpot/blob/master/Boilerplate/CustomMenuPrimary.html
Hope this helps!
If we were able to answer your query, kindly help the community by marking it as a solution.
Thanks and Regards.