With one file:
write/binary %test.tar tar %my-file.txt
With a block of files:
write/binary %test.tar tar [%my-file.txt %my-dir/my-file.bmp %just-a-dir/]
You can of course gzip the tar (with %gzip.r):
do %tar.r
do %gzip.r
write/binary %test.tgz gzip tar [%some-files ...]
Resulting archive is usually smaller than a *.zip of the same files.