SourceTableTemporary - Modify record in real table

Hi everyone,

I want to run a popup page from the Sales Order page when entering a Sell-to Customer No.
In this page I can fill in some fields (Adress, "Address 2", "Post Code", etc.) and when I click OK the values I entered in the popup will be in the Sales Header(the values from Customer table are not taken).
I created a list page with the fields I need from table 36 and set the property SourceTableTemporary to Yes.
In the OnOpenPage trigger I placed the following code:
IF INSERT THEN;

I added a function executed in page Sales Order when entering a Sell-to Customer No with the following code:

CLEAR(pageXXX);
pageXXX.SETTABLEVIEW(Rec);
pageXXX.SETRECORD(Rec);
pageXXX.LOOKUPMODE :=TRUE;
pageXXX.RUNMODAL;

When I get the popup page, I fill in some data and click OK, there is no error message or so but the Sales Order is not modified.
What shall I change in my code to get the result I need?

Thanks in advance