./program --input-file $(ls -1 /tmp/file*.txt)
The -1 option is unnecessary and it is safer to ensure ls is not an alias (it usually is!) giving unexpected output, leading to
./program --input-file $(/bin/ls /tmp/file*.txt)
That will work as long as you don't have whitespace characters in the /tmp/file*.txt names.
Sorry, I misunderstood your requirement. Now I understand ./program requires a file but you don't want to make one. Hmm! Not as self-contradictory as it sounds if ./program is written to follow the *n*x convention of accepting filename "-" (without the quotes) as meaning standard input.
If it does then this will work
echo $(/bin/ls /tmp/file*.txt) | ./program --input-file -
It all depends on how ./program is written.