Adjust your popup layout to modal:
and contribute on the documentation on github: https://github.com/mendix/docs/blob/development/refguide6/show-page.md
read here to see how: https://docs.mendix.com/community/documentation/contribute-to-the-mendix-documentation
That might be old documentation that hasn't been updated yet...
In recent versions of the Modeler, whether a page is a blocking popup or not is defined in the Layout used. Create a copy of your Popup layout and change the Layout type to Modal pop up, then use that layout for your form.
Ok this is getting stranger, i re-installed the modeller and its still persisting, also i changed the navigationlayout to Model pop-up and still i cant find Location at my show a page activity.
EVEN STRANGER: i asked a colleague to try to do the same thing in the same project with both 0 changes and both latest version from the teamserver and same Mendix 6.10.0 version and he WAS able to change the activity to blocking pop-up.
Thanks for your quick responses but this has not helped with the problem..
OK its solved, after my colleague made some changes and a commit and i did an update it worked again.