How to insert the header No. in the subform

Hi everyone,

I am trying to create a new page with a header and a subform.

In the source table of the subform I created a field named "Group No." and I set the TableRelation to "myTable"."No."

In the header page I set the  SubPageLink to "Group No."=FIELD("No.").

When I insert a line in the subform, I look at the table and find the field "Group No."(which is supposed to contain the header No.)

empty....

What shall I do to populate this field?

I looked into tables 36 and 37  with pages 42 and 46 but couldn't figure it out...

Can anyone help me on how to proceed???

Thanks in advance  :)