Insert caret just before specified character

How do I add caret '^' before each comma ',' in text string? Trying this:

echo {"errors":{"subject_id":["mandatory","Contact not exist."]}} | sed.exe -e "s/,/^,/g"

but does not work. Also trying this:

echo {"errors":{"subject_id":["mandatory","Contact not exist."]}} | sed.exe -e "s/,/^^,/g"

because in windows shell caret is escape character. So in this case first carret should escape another one (which should remain), but it also does not work.

What am I doing wrong?

L.