I am looking for a way to automatically have certain tag fields be automatically filled on start with certain information. For instance, if I am tagging 17 audiobooks in 17 different folders from the same author, I would like to have the "Artist" and "Albumartist" fields automatically have "Jim Butcher" filled in, even when I refresh the directory. As it stands now, every time you add new files or change directory, it reads what is in the new group of files and puts whatever is there in the tagging fields. I want to be able to set some of those fields to ignore this and put user custom text in there automatically right from the start, that way I don't have to type in "Jim Butcher" 17 times for each group of mp3s, and can just edit the Album field. I think this used to be possible but I can't find this function in the program anymore. Am I just missing it or did it get removed?
I do not quite understand the workflow: If you load all the files that should get the same information, it is possible to save the same data to all these files in one go. No need to repeat it 17 times.
And yes, there are no standard values for fields any more (have a look at the changelog).
[2018-05-07] CHG: removed default values configuration.
You have to write actions of the type "Format value" to assign a standard text to fields.
It just came back 
Starting from Mp3tag v2.90c, you can now specify a default value per field on the Tag Panel via "Options > Tag Panel".