A Popover is just and extension of the layout. I should be treated as such. it does not require it's own TOG. If you are looking to add info to a portal the popover button can be in the portal line.
Your way of doing is workable but need a bit more work. For me, I will create a set of global fields (in the Work Order table) and place them in the popover. User enter note details in these global fields . When user hits the Add Note button, you use script to create the portal row. You need to do some housekeeping like clearing those global fields and so on.
The other method as mentioned by cwiltgen86, you put the popover button in the portal row. In the relationship, turn on the "Allow creating of this table via this relationship". The fields in the popover come from the Note TOC. If the portal row is empty (meaning the Note has not been added) , all the fields in the popover will be empty. Once you enter data into those fields, the portal row will be created automatically for you (via the relationship). I created 2 popover. One with Add label and the other with Edit label. I used the Hide object to hide one of the popover. If the portal has not been created, the Add button is visible. If the portal row has already been created, the Edit button is visible...