Good Day,
I have a table called Purchase Orders and then another called Invoice Lines.
These two tables are related by a One-to-Many relationship (One PO - Many Invoice Lines).
On my portal I want an administrator to be able to Create a new PO and the Invoice Lines in a single go.
Form View is as below:

When I preview the form I don't see the option to add new Invoice Lines.

I have created the permissions and assigned them as per the below:

Nothing special about the view... I have the New PO and then added a subgrid below for Invoice Lines.

Clearly the Relationship is working as you'll not some of the existing Line Items pulled through to the above view.
Again, not looking for anything special here.... I'd expect this to work OOB.
Any assistance?
Thanking you in advance.