Federated-Identity POST API

I was wondering if anyone was able to help I am trying to make a POST request for federated-identities.

Currently use the code below:

$body = @{
    "userid"="12345..."
    "username"="username"
}

$restResults = Invoke-RestMethod -Uri "https://$url/auth/admin/realms/$realm/users/{user-id}/federated-identity" -Method Post -Headers $Headers -Body $body -ContentType "application/json"

But the only response I get is

{“error”:“RESTEASY003650: No resource method found for POST, return 405 with Allow header”}

I’ve checked the API documentation and this looks like it should be possible, if anyone can give me some direction it’d be appreciated, thank you.