You would have to use an action of the type "Format value" or Convert>Tag-Tag for that field.
Ugh, yeah the whole point is that I wanted to create an entry in the Custom List Values for the Encoded by field. This way I could easily select it from the drop-down list and be able to set my Encoder Source along with the Current Date, i.e. Encoded by: qaac -v256 -q2 2024-06-10...
I wonder if Florian would consider adding this in a future version of Mp3tag or I wonder if there is a reason he hasn't already added this? Like maybe some other limitation I'm not thinking of?
The Tag Panel, and by that also Custom List Values, is one of the areas where format strings and scripting functions are not supported. The main reason is, that by enabling that users need to be aware of Characters with Special Functionality when editing, which is beyond simple for someone who uses Mp3tag for editing just a few files.
Another reason are possible cycles where one field references another which references the original field. This is not a hard problem, but adds complexity to support a feature which is elsewhere already perfectly possible.
In your case, you'd just create a new Action Group with an action Format Value for ENCODEDBY using qaac -v256 -q2 %_date% as format string. It's then available from the actions menu and the menu of the actions button in the toolbar.
You can also use %_tool% %_date% for MP4/M4A/M4V files to use the actual value from the ©too atom.
I had a feeling that was probably gonna be the reason why it hadn't been implemented already. It's not a big deal, I created an Action to do the job and it works just fine. Probably even faster than using the drop-down menu like I was originally planning to do. Thanks for the explanation and the suggestion. 