Please ask a more precise question.
Is your question about doing it programmatically or how to set mail display ? If the latter, that's not a developer forum question but a question to product support.
if it is for doing it programmatically:
Is it a MacOS question ? Then this is not the best section of forum, should be Cocoa.
Why specifically for 10.15 beta ? Did you notice it did work before and no more on 10.15 ?
what is exactly your problem: what have you tried so far, what do you get or don't get ?