After finding a description of the process in the Subaru service manual I ended up using the vehicle's scissor jack to lower the rear arm to meet the lower bolthole on the strut. The guide advised putting the jack upside down between the link rear and sub frame. The diagram didn't help me so I took a guess and saw only one spot that made sense and ...
Use a floor jack on the bottom of the shock. A few bags of cement / sand in the trunk of the car stop it from rising up on the strut.
It's advisable to make some sort of V-shaped support bracket to fit into the top of the jack - you don't want that strut slipping off when compressing it.