How do you set a Sekonic Light Meter to "ambient"?

Which version do you have? I have used the DuelMaster L-758Cine. It automatically took “ambient” as the default (Although it was not my light meter, so the person who owned it could have had it set to ambient, as default.) Most light meters I have used automatically consider ambient as the default and allow you to change up from that

If the white ball thingy is over the eye, it measures ambient. If the white ball thingy is next to the eye it measures incident light.