It’s a bit odd, but once you’re at https://musicbrainz.org/cdtoc/attach?id=ON6Cj.RfASOHrqvmPah4w5ajp4Q-&tracks=7&toc=1+7+216825+150+32531+53532+80052+128560+148034+180879&tport=8001 start your search by artist or release, for example search by “Robert Schumann” and it will give you something like https://musicbrainz.org/cdtoc/attach?toc=1+7+216825+150+32531+53532+80052+128560+148034+180879&filter-artist.query=robert+schumann
Now comes the part that screws most people up. Select the radio button next to the desired name and NOT the name link itself. For example, selecting the radio button for the first name on the list (the composer) and clicking “Select” brings up https://musicbrainz.org/cdtoc/attach?toc=1+7+216825+150+32531+53532+80052+128560+148034+180879&filter-artist.query=robert+schumann&artist=32513 which is a list of all the releases which match the number of tracks on the media you’re trying to link. Now you can select the radio button next to the desired release / media and click the “Attach CD TOC” button at the bottom of the page. If you can’t find it in the list, you have the option of adding a new release.