Transition JIRA issue via API returning a 400

I'm trying to transition issues via the API using .NET but I'm consistently getting a 400 error back. I'm wondering if anyone can see anything obvious that I'm doing wrong?

Code:

string example = @"{
""id"": ""221""
}";

string ticketjson = JsonConvert.SerializeObject(example); 

string postUrl = "https://myurl/rest/api/2/issue/" + issueKey + "/transitions";
System.Net.Http.HttpClient client = new System.Net.Http.HttpClient();
client.BaseAddress = new System.Uri(postUrl);
byte[] cred = UTF8Encoding.UTF8.GetBytes("username:pwd");
client.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Basic", Convert.ToBase64String(cred));
client.DefaultRequestHeaders.Accept.Add(new System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/json"));

var content = new StringContent(ticketjson, Encoding.UTF8, "application/json");
var response = client.PostAsync(postUrl, content).Result;
if (response.IsSuccessStatusCode)
{
string result = response.Content.ReadAsStringAsync().Result;

return result;
}
else
{
unauth.Text = "There has been a problem submitting your comment. Please try again.";
return response.StatusCode.ToString();
}

The transition exists and when I go to the postUrl I can see it:

enter image description here

Any help would be appreciated!

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