Should you fit door below damp proof course?

Hi, hoping for some advice as I can't really find much discussion along these same lines...

I've picked up a 2nd uPVC door to replace my metal door/frame for side access to garage.

The existing door is basically sat on the top of the brick one layer down from the damp proof course, which is effectively level with the surrounding patio.

I'm replacing partly because the existing set up shows damp coming through on the floor by the door when it rains with an easterly wind.

I don't know how I should fit the uPVC frame - straight onto the brick? Try and seal underneath? On a thin bed of mortar and add some dpm before the frame?

I'll try and attach some pics - any thoughts welcome! Thanks

 

I might ask this again but purely "should I fit uPVC door on a bed of mortar"
I'm guessing the answer is yes with packers under there too, but it's just the damp thing that worries me...
I'm not sure how I'd stop it making it's way under the door effectively as there isn't a "wall" because the door opening is cut low for whatever reason