The api_* parameters need to be in the query parameters, not the POST body.
So you need something like:
fetch('https://community.testsite.com/admin/users/sync_sso?api_username=USERNAME&api_key=KITTENS', { method: 'POST', body: body })
The api_* parameters need to be in the query parameters, not the POST body.
So you need something like:
fetch('https://community.testsite.com/admin/users/sync_sso?api_username=USERNAME&api_key=KITTENS', { method: 'POST', body: body })