hi @fengzd
Is there a particular reason why you have chosen to go with sslforfree.com?
There are plenty of good windows clients
LE64.exe
letsencrypt-win-simple
ACMESharp
Certify
I would have thought using one of these on your IIS server would make more sense?
I would try Certify first as it has a GUI and is pretty intutive
Andrei