There is no such option, you just import the exported ortho to QGIS.
You need to use this corner (the selected corner is basically upper left corner of the ortho)
Regarding to the heights of the ortho check these: About laser scan reference - #48 by Anonymous_b53149a1f1, better way to export orthophoto to cad software - #10 by Anonymous_9a96793ccc, Orthophotos - Mauvais Z, tête en bas, différentes altitudes pour une même bôite... - #2 by OndrejTrhan
If you are exporting the georeferenced ortho, the Z values are there, you just need to use one plugin for AutoCAD (it is mentioned in the attached comments). Then you will be able to measure the height of the bottom left-hand corner.