Using the /summon minecraft:wolf ~ ~ ~ {Sitting:1b} command you can summon a sitting wolf, but it can still walk. You cannot let the wolf stand up until tamed.
A few hours after I made this, I realized this was duplicated from MC-77841.
Environment
Windows 10 Home Version 1903
Processor: Intel(R) Core(TM) i5-4210U CPU
Build: 18362.476
Yep, duplicates MC-77841. Intended game behavior as the wolf doesn't have an owner yet.