"assignments": {
"d2817eda-ce46-42fe-89f4-525ce2823b45": {
"@odata.type": "#microsoft.graph.plannerAssignment",
"assignedBy": {
"user": {
"displayName": null,
"id": "34651387-a6c8-4459-922b-575b60df3ede"
}This is the JSON output of a List All Task functions in regards to assignment, looks like Planner uses the Microsoft Graph API to generate ID's.
So you will need to either authenticate with Oauth and generate the ID or get the output from "List all tasks" figure out which ID is related to which user (very unscalable).
I have only just started today on Planner Flow's so there is likely an easier way, maybe this is enough for someone else to answer the question. I will be spending more time on this tomorrow, if I find something around this I will post back.
If you feel like reading check the Graph API documentation here https://developer.microsoft.com/en-us/graph/docs/api-reference/v1.0/api/user_list