using var client = new HttpClient();
client.DefaultRequestHeaders.Add("x-api-key", "YOUR_API_KEY");
var content = new StringContent(@"{
""emailAddress"": ""string"",
""firstName"": ""string"",
""lastName"": ""string""
}", System.Text.Encoding.UTF8, "application/json");
var response = await client.PostAsync("{{API.SANDBOX_BASE_URL}}/IdentityVerification/ReverseEmailLookup", content);
var result = await response.Content.ReadAsStringAsync();