I need to create a workflow for a custom list (having two columns Title and Hyperlink). When an item is created in that list the workflow should ceate a corresponding item in a discussion list with the same Title and the Hyperlink field in the first list should be updated with the URL of the discussion item.
The Create List Item workflow action, under List Actions, returns the ID of the created item. You can store this in a workflow variable.
Then set the Hyperlink field to a concat string with the fix part like
http://myserver/sites/a/lists/mylist/DispForm.aspx?ID= plus the variable.
You can be little more clear about your question. As far as I understood... You need to create an item in discussion board, when item is created in the custom list. For implementing it you can use the action 'Create List Item' in SharePoint Designer workflow which creates list item in another list.