Custom text in header texts tab

[Dan3]

Hello experts, 

I would like to do something, but I am not sure how to achieve it and I would really appreciate some help. Whenever a sales order is created and the shipping point is '1000' let's say, I want a custom text to appear in header->Texts if the shipping point is 1000 so the user doesn't have to enter it manually all the time. I know what is the Text type.  

I have already tried to use userexit_save_document and the FM save_text. The problem is that even if I have the VBELN to pass to the FM, the document itself is not created at that point so no text in written. If I execute the same FM in se37 on an existing document, the text is changed. I also tried to look into VOTXN and VOFM, I notice that there is a routine for my text type, but I am unable to find it to modify the code.

I am open to solve the problem with either ABAP or customization, but unfortunately I don't know how. Any suggestion would be really appreciated. 

Thank you!