This is probably mistaken, according to the docs of @add@:
"@remove@<propname>=id(s) or @add@<propname>=id(s)
    The “@add@” and “@remove@” edit actions apply only to Multilink
properties. The form value must be a comma-separate list of keys for the
class specified by the simple form variable. The listed items are added
to (respectively, removed from) the specified property."

So the value should really be an id not a designator as given in the
example. If a designator is available @link@ should be used.

Now maybe the submitter originally experimented with both @link@ and
@add@ -- I've just committed a fix that permits linking an already
existing node to a newly-created one (e.g. an existing msg to a newly
created item's messages property).