Dear Adrian,
We don't have the possibility to "interact" with Google Earth in such a way, so obtaining the coordinates from a point in the map though a mouse=click mouse-click is not doable.
In the Plots of the Collect Earth surveys we only allow one Location attribute (the central position of the plot) so if you want to write a coordinate by hand I would recommend to add two number attributes (of type real) and collect the latitude and longitude separately.
We will keep in mind this feature for coming versions of CE but for now the "two field" coordinate solution will have to do.
Regards! Open Foris team