The aforementioned
Style Guide is on Tutorials, [EN]Creating New Title Pages. Links are normally put in their sections, synopses are not meant to be of links (and you can add one synopsis per language) but some titles have one in special cases. From what you're describing you want to put a link where we can find the original doujinshi, right? If it is official (Pixiv, author's page, etc.), put it on "Official Sites" and "Raw", however if it's from elsewhere (for instance e-hentai, your group webpage, Internet Archive, etc.) the link should not be added anywhere. In fact, it is not allowed to post links to aggregators or pirate sites (such as torrent pages) if that's the case, otherwise you can always put it in on the comments or the credits page (just make sure it is not uploaded on a forbidden site per
MangaDex rules).