how to setup x-default markup ?

Hello testwp75,

in the Hreflang -> Connections menu select “x-default – don’t target a specific language or locale” in one of the Language fields. It’s the first option of the selector.

If your question is more on the technical implementation of x-default in general, please see this document:

https://developers.google.com/search/docs/advanced/crawling/localized-versions#xdefault