Unfortunatelly, it looks like Planner REST API should add to Task definition related Link parameter first so Microsoft Flow could offer this piece of info. I mean, I have been reviewing Planner REST API and did not find a Link parameter in Task Object definition. If my understanding is wrong, please feel free to correct me, since I also see extremely useful to have Task related Link available, i.e. in the same way items from a Sharepoint List have this parameter available.
In the meantime, I found a post describing a potential workaround for this limitation, by building the link following the format: