How to set up DKIM canonicalization to "relaxed"?

Hi, this issue has been driving me nuts for a day. I have SPF and DKIM set up correctly. However, only "DKIM Signature Body Hash" fails verification. After tons of research, it turns out I have "Header Method" and "Body Method" set to "Simple". I would like to change it to "relaxed" and I can't find out how or where.

Essentially, in the DKIM Signature, I would like to have "c=relaxed/relaxed" instead of "c=simple/simple".

Anyone knows how to do that?

Much Appreciated.