How do i generate unique data in a loop test?

I have my loop test set up to run my test multiple times successfully. However, I need each concurrent run to generate a unique "message ID". I have a "DataGen" test step with a "Number" configuration that I have set to increment starting from 300 and ending at 400 but it doesn't seem to increment within the loop. It will increment every time i start the loop over though. I think i am probably approaching this wrong. I need just 1 test step to run 100 times in a loop but i need my message ID to be unique for each of the 100 runs. If anyone has any ideas I would greatly appreciate it.

thanks,
Jake