Sveiki, Jānis! Look for this links in form designer on the "Command interface" tab. Theese links are Common command or Report command or Data processor command. You may turn on/off visibility of links by checkboxes.
Sveiki, Jānis! I read carefully your post and understand that you designing a document rēķins (invoice or счет-фактура) and you need to fill tabular section with products from selected purchase document by press "Add" button. Right? There's nothing easier) You must add tabular section ("Products" for example) in your document rēķins, that will contain all fields you need. Also add procedure, that get data from Owner and fill tabular section with values from Owner. For example:
Unfortunately, there's no way to use tabular section of other document and link additional data to it. If you generate document based on other, you must copy required data from it. Rows of document tabular sections have no refs to link from the outside (don't use LineNumber, 'cause it changes sometimes and you lose your link).