As discussed, the URL identifier should work. But, as it happens, I recently created something that might be better in this situation.
There are sites out there that have books, but, doing a metadata search on them doesn't make sense. The ones I am thinking of are sites were people post stories. Some will have epub or mobi available for downloading, and others are covered by tools like FanFicFare. And there is the library here at MobileRead. In each case, if you want to record the link to where you got the book, you need to add a URL identifier. Or write a metadata source plugin. I have done the latter a couple of times, and it has always felt like overkill. But, I like having the site name displayed rather than the domain name.
Out of that came the "Generic Identifiers" metadata source plugin. With this I can define an identifier, it's name and the URL to use. Some examples will show what I mean.
MobileRead:
Code:
Identifier: mobileread
Display Name: MobileRead
URL: https://www.mobileread.com/forums/showthread.php?t=%s
Smashwords:
Code:
Identifier: smashwords
Display Name: Smashwords
URL: https://www.smashwords.com/books/view/%s
Baen (but use the Baen/Webscription plugin):
Code:
Identifier: baen
Display Name: Baen
URL: http://www.baen.com/%s.html
In each case, the "%s" in the URL is replaced by the identifier value when the link is built by calibre. If there is no "%s", the value is appended to the end of the URL.
The plugin is attached for anyone who wants to try it. I hadn't decided if I was going to put it out, but it should fit what Iouanalao wants to do. If there is enough interest, I'll create a plugin thread for it. And if I do, suggestions for a better name are VERY welcome.