I found a workaround for that, I defined the datatemplate in the page.resources and assigned that in the code behind like this:
myPopupLayout.PopupView.ContentTemplate = myTemplate
and I had to assign the Title, button texts etc. like this too, but its inconvenient.
The documentation seems to be wrong or deprecated, because I cannot set the PopupView property neither in code behind, nor in xaml, like this:
I switched to the definition of the SfPopupLayout, and found out that the PopupView property is write-protected: (see screenshot in attached zip file)
Is that a bug or is that intended? I'm using version 220.127.116.11. I see there is a newer version (18.104.22.168) I just updated my Syncfusion nugets three days ago, maybe that update will fix it.