You can’t. Roon can create “Roon Tags” from special file tags, namely ROONALBUMTAG and ROONTRACKTAG. The link to the relevant document of the Roon manual was given in the release note directly below the text you cited. There is no support for arbitrary user-defined tags to do this.
The use of ROONALBUMTAG or ROONTRACKTAG is an additional method that wasn’t present last time, unfortunately it still does not seem to be what you are looking for.