That's how you need to fill it, 123456IMM.I did enter ECA report number from my report for both of my degrees. But i have one doubt about it though, that my actual Reference # is only a number like 1234 as i see it in my WES online account, BUT the report shows few alphabets at the end of it like 1234IMIM. I have typed it with alphabets as is in my report.
Not sure if this makes any difference.
Did you choose degree awarded for the status of education?