Updated with solution:
Here's the successful parameters of the Office 365 groups Send an HTTP request V2 action:
Method: POST
URI: https://graph.microsoft.com/v1.0/groups/{id}/owners/$ref
Content-type: application/json
Body:
{
"@odata.id": "https:
}
I've built a flow that creates security groups and adds members to it but I can't seem to figure out how to add an owner.
From what I've found, I'll need to use an HTTP request and Bing provided this answer

which I think has potential to work but I'm not sure what to replace "token" with.
Has anyone successful created an action to do this?