To add an external URL to a button in the hub menu, enter the Super-> Company Menu Management
Click on the menu you want to edit. When you see the correct hub menu- click on the edit icon in the upper right corner.
Tap the field+ if you need to add a new field. Then click on the folder icon , and you will get up a menu where you have the option to add External URL, press it and enter the info you want.
Save and then the open link you inserted is connected to the field you added it in.