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I think the API reference is incorrect. I got the same error when assigning the host extension id that way. Let me talk to the documentation team.

Meanwhile, this way works well for me. I don't know what programming language you are writing but this snippet is in Node JS

function create_meeting(extId)
    var endpoint = "/account/~/extension/~/meeting"
    if (extId != "")
        endpoint == "/account/~/extension/" + extId + "/meeting"
    platform.post(endpoint, {
              topic: 'Test Meeting',
              meetingType: 'Instant',
              allowJoinBeforeHost: false,
              startHostVideo: true,
              startParticipantsVideo: false
        })
        .then(function (resp) {
            console.log( 'Start Your Meeting: ' + resp.json().links.startUri )
            console.log( 'Meeting id: ' + resp.json().id )
        })
        .catch(function(e){
          console.log(e.message)
        });
}