I've been very grateful for a number of posts elsewhere on the forums for advice on setting these tabs up.
After experimenting for a while to see what is possible, I've settled on strings which work for me. In general, I derived these by running a normal browser session and searching for a given artist and album. In most cases, I think you can copy the string from the browser's address bar, and edit it to replace the specific names with MusicBee tags (such as <Album Artist> and <Album>) before pasting the result into the Displayed Tabs set up dialog.
I've pasted my current settings below in case they are useful to anyone else experimenting with these tabs ......
Amazon UK
http://www.amazon.co.uk/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=<ASIN>
Note that I store Amazon's ASIN value in a tag on my files, which makes it very easy to find the album/track - assuming it's still available. Amazon seems to handle the search reasonably elegantly when I've not got the ASIN stored locally, or if it cannot be found.
Discogs
http://www.discogs.com/search/?q=<Album Artist>+<Album>&type=all
Music Brainz
https://musicbrainz.org/taglookup?tag-lookup.artist=<Album Artist>&tag-lookup.release=<Album>
AllMusic
http://www.allmusic.com/search/all/<Album Artist> <Album>
I guess these strings may well need to change over time as external web-sites modify their interfaces, and I don't know whether it makes any difference if you're based in a different country to the UK (where I am).
My thanks to all who share their knowledge by posting in the forums - I've learnt a lot from their generous advice.
Good luck & successful experimenting, Romney