Songwriter tags for flac files

I am a newbie, so please bear with me.

My music collection is in flac format and tagged with mp3tag.
I wish to add songwriter tags (multiple for some tracks).
This is to have the tags read by JRiver Media Center so that I can play tracks written by a specific songwriter eg. Irving Berlin.

I have a database (Music Collectorz) where I have painstakingly input songwriter data. Such data can be exported.

The question then is: What import format/syntax is recognized by mp3tag so that I can import the songwriter data into the songwriter tags?

Many thanks in advance.
Best regards

See the help on the Convert>Text File-Tag function:

It's mainly plain text. You have to specify on import how the data is structured and which fields should be filled.
Which brings me to the problem:

Have you got any track at all that shows the songwriter in this player?
If so, then let MP3tag have a look at this file and open the extended tags dialogue (Alt-T) to see the field name.
Otherwise it is not always granted that every player displays every field for every format.
Perhaps the list of supported fields helps:

Many thanks ohrenkino. Much appreciate your help and advice.

Sorry I did not immediately respond to you.

I took the time to follow up on your suggestions and after evaluating the pros and cons, it appears that I will have to use the Composer field for songwriters inserting a semicolon when there are multiple values.

The composer tag is also recognised by JRiver so that it makes things easier for me this way.
Best regards and thanks very much once again.