create a value list using values from your "pkShowroomID" field.
Then in layout mode in your portal in the products layout (and also if you'd like in your Join Table layout), select the "fkShowroomID" field and either via the Inspector or by right clicking on the field, set that field to use the newly created "pkshowroomID" value list.
The values will now be automatically updated and contain all the values.
Here's a link to a demo file that does a many to many relationship to match Contracts to Companies. You can download it (click the blue download button) to see an example of what rdowler is recommending here: http://www.4shared.com/file/PLhjErzu/Contracts_to_Companies.html