Hi there,
From what I understand you want to add a new button (for a new page) in the primary menu.
If that is the case then you should go to:
appearance -> customize -> menus -> primary menu -> add item (which will be the button you need to add). You should see something like this: https://prnt.sc/k1xm98. If this is not what are you asking for, please give us more details about what you want to do.
Regards,
Silviu