Creating mail subfolders in Big Sur

I’m having trouble creating a mail subfolder with applescript in Big Sur (account is on Exchange)

Suppose I want to have a top-level mail folder “foo” with a subfolder “bar”
I try:

tell application “Mail”
set newbox to make new mailbox of account “MyConnect” with properties {name:“foo/bar”}
end tell

This strangely creates the top-level folder “foo” and a sub-folder called “foo/bar” within foo (i.e., with a “/” in the name). Does the same thing whether or not “foo” is already there.

Any help would be appreciated.

Thanks! Mark