Right now you can make a fixed header, but it will be fixed from the start. From what I understand you want it to become fixed after the page is scrolled a bit. There's no built in functionality for this, but there is sort of a workaround for it.
You can do this by making 2 versions of the header:
- One default version, when you are at the top of the page, that is not fixed.
- One "sticky" version, that is usually smaller and is fixed to the top of the page.
You can put the big header on top, so that it hides the smaller header. You can also use a cover rectangle that will cover the smaller header until it has to show up. On the smaller sticky header, you activate the "Fix position while scrolling" option on the right. This way the top header will scroll with the page, and the sticky header will show under it after you scroll.
I made an example in this thread for another user, you can check it out:
https://community.adobe.com/t5/adobe-xd/fixed-position-when-scrolling-until-a-certain-point/m-p/11467208?page=1#M37629
Hope that helps.
Cheers!