Here's a powershell script I use to do same. I'm sure you can work it all out from that, even if you are not using powershell, it has the api endpoint (note it must be https) the full url to point to create service token operation, the body of the request and the header for content type for json format. It then extracts the token value from the response.

function get-trkdapiToken($app, $username, $password)
{
   $trkdapi_url = "https://api.trkd.thomsonreuters.com"
   $trkdapi_tm_url = "$trkdapi_url/api/TokenManagement/TokenManagement.svc/REST/Anonymous/TokenManagement_1/CreateServiceToken_1"
   $trkdapi_tm_body = @"
   {
        "CreateServiceToken_Request_1":
        {
            "ApplicationID": "$app",
            "Username": "$username",
            "Password": "$password"
        }
    }
"@

   $w = invoke-webrequest -Uri $trkdapi_tm_url -Body $trkdapi_tm_body -ContentType "application/json" -Method Post
   $b = $w.Content
   $j = $b | ConvertFrom-Json
   $r = $j.CreateServiceToken_Response_1
   $t = $r.Token
   $e = $r.Expiration
   return $t
}