Here is a similar case - not an official webpage, but a page that explains the background of a particular song and I would like to integrate the link.
Diane Tell explains on her website that her song Reste avec moi (Bonheur d’occasion) is a cover of of a melody by François Dompierre, which was a theme song in the film Bonheur d’occasion.
That theme song was not yet in MB, so I added it to MB, then added it as a related work to Diane Tell’s recording on album Passé Simple.
I would want to add the link itself “Reste Avec Moi (Bonheur d'occasion) - 1983 / 2013 - Diane Tell, site officiel” to the recording page, but the options don’t forsee that:
I added the link to the annotation field of the album instead.