How to set the column "Related Item"?

Hi,

I have a document library and a discussion board. For both I have activated the column "Related Item".

Now, with a workflow started manually at a document level, I want to:

* create a new discussion in the list "Discussion Board"

* set the column "Related Item" with the URL of the document

Well it seems easy but I'm not able to manage to set the "Related Item" column.

Any idea?

Thanks