How do I make a field from a tag source

Step-by-Step:
1.) Load your album - let's say your 12"-Vinyl-Album with 10 Tracks called "The Final Countdown" from Europe - into Mp3tag

2.) Select all 10 tracks

3.) Call your Tag Source Script and choose the exact same album from Country "XE"

4.) If you see the same 10 track names on the lower right side - exactly aligned with your local 10 tracks on the lower left side - then press OK in the Tag Source result window
-> This saves all the visible information from MusicBrainz into the tags of your tracks

5.) Use the Convert Tag -> Filename (ALT + 1) and build your new filename as you want it, for example:
image
Format string:
%ALBUMARTIST% - %ALBUM% - %YEAR% - (%MEDIATYPE%)

IMPORTANT For those finding this solution later:

If you want to have unique tracknames you could try a format string for Convert like this:
%ALBUMARTIST% - %ALBUM% - %YEAR%[ - ($replace(%MEDIATYPE%,'"',''''))]\$regexp(%TRACK%,'(.*)\/(.*)',$1) - %TITLE%

This would create the following Vinyl track name #1:
Europe - The Final Countdown - 1986 - (12'' Vinyl)\A1 - The Final Countdown.mp3

Of course you can replace the invalid " character in MEDIATYPE with whatever character(s) you like. Examples are inch or the two '

The regular expression after the \ (= backslash, separating the directory name from the filenames) extracts the part before the / (=slash, separating the tracknumber from total number of tracks in this album) in %TRACK%. Otherwise you get a A110 as Vinyl track number.

If you prefer to see a numerical tracknumber 01 instead of A1 you can use this format string:
%ALBUMARTIST% - %ALBUM% - %YEAR%[ - ($replace(%MEDIATYPE%,'"',''''))]\$num(%_counter%,2) - %TITLE%
Please be aware that your track B1 will become 06 if it is the 6th track in the selected list of tracks.