Getting User based bearer token

MrCoups 

I'm trying to call this "https://graph.microsoft.com/v1.0/me/onlineMeetings" api. To call this, i need user based bearer token.

Here is reference "https://docs.microsoft.com/en-us/graph/api/application-post-onlinemeetings?view=graph-rest-1.0&tabs=http"

Here is what i try

private string AddOnlineMeeting(string accessToken,string meetingName)
{

string webUrl = string.Empty;
try
{
var data = new
{
startDateTime = "2020-07-13T14:30:34.2444915-07:00",
endDateTime = "2020-07-14T14:30:34.2444915-07:00",
subject = meetingName
};

var serializer = new JavaScriptSerializer();
var json = serializer.Serialize(data);

WebRequest createGroupRequest = WebRequest.Create("https://graph.microsoft.com/v1.0/me/onlineMeetings");

createGroupRequest.Method = "POST";
createGroupRequest.ContentType = "application/json";
createGroupRequest.Headers.Add("Authorization", "Bearer " + accessToken);
byte[] byteArray = Encoding.UTF8.GetBytes(json);
createGroupRequest.ContentLength = byteArray.Length;
Stream dataStream = createGroupRequest.GetRequestStream();
dataStream.Write(byteArray, 0, byteArray.Length);
dataStream.Close();

using (WebResponse response = createGroupRequest.GetResponse())
{
string jsonR = string.Empty;
using (StreamReader reader = new StreamReader(response.GetResponseStream()))
{
jsonR = reader.ReadToEnd();
}

CreateMeetingResponse meetingRes = JsonConvert.DeserializeObject<CreateMeetingResponse>(jsonR);
webUrl = meetingRes.joinWebUrl;
}
}
catch(Exception ex)
{

}

return webUrl;
}