As I see from the guide there you may "specify file names, you can include them as the last arguments on the command line, as follows:
tar OPERATION [OPTION1 OPTION2 ..] [FILE NAME-1 FILE NAME-2 ...]"
Perhaps AmigaDOS command LIST with argument LFORMAT may be usefull there to produce list of *.txt files at first and then such output to be used as argument for TAR then...
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