Microsoft Teams service API (create/edit) doesn't work

Summary

Reproducing steps described at https://docs.gitlab.com/ee/api/services.html#createedit-microsoft-teams-service doesn't lead to Microsoft Teams service creation. Self-hosted GitLab Enterprise Edition 12.9.5-ee.

Steps to reproduce

curl --header "Private-Token: $glnswToken" "$gl/api/v4/projects/openmgw%2FSS_MGWLogMon/services"
[]
curl -X PUT -d '{ "webhook": "https://outlook.office.com/webhook/4295accc-f365-4f74-ba9d-63f0a75ab4f0@5d471751-9675-428d-917b-70f44f9630b0/IncomingWebhook/b92ae1d457344244a02e241cbde0914c/2c2c80a5-2f2b-4edf-8d99-a73484f78c6c", "notify_only_broken_pipelines": true, "branches_to_be_notified": "all" }' --header "Private-Token: $glnswToken" "$gl/api/v4/projects/openmgw%2FSS_MGWLogMon/services/microsoft-teams"
{"error":"webhook is missing"}

What is the current bug behavior?

{"error":"webhook is missing"}

What is the expected correct behavior?

Microsoft Teams service integration creation (so it works like it was created via Web UI).

Relevant logs and/or screenshots

N/A

Output of checks

N/A

Results of GitLab environment info

N/A

Results of GitLab application Check

N/A

Possible fixes

N/A