Dynamics 365 CE (CRM) Web API Authentication by using OAuth Client Credentials Grant

d365_webapi_authentication_by_using_OAuth_ClientCredentials.cs

public string RetrieveAuthToken(string authorityURI, string d365Url, string clientId, string clientSecret)
{
    string result = string.Empty;

    using (HttpClient httpClient = new HttpClient())
    {
        string tokenUrl = $"{authorityURI}/oauth2/token";
        HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post, tokenUrl);

        request.Content = new FormUrlEncodedContent(new[] {
                    new KeyValuePair<string, string>("grant_type", "client_credentials"),
                    new KeyValuePair<string, string>("client_id", clientId),
                    new KeyValuePair<string, string>("client_secret", clientSecret),
                    new KeyValuePair<string, string>("resource", d365Url),
                });

        var response = httpClient.SendAsync(request).GetAwaiter().GetResult();
        var content = response.Content.ReadAsStringAsync();

        var parsedData = JValue.Parse(content.Result);

        result = parsedData["access_token"].ToString();
    }

    return result;
}

This code snippet assumes "authorityURI" parameter is like https://login.microsoftonline.com/{TENANT_ID_GUID} and add "oauth2/token" suffix at the end of this URI.