posttoteams.cs

public async Task<ActionResult> PostToTeams(string teamId, string channelId, string message)
{
    var httpContext = _httpContextAccessor.HttpContext;
    httpContext.Request.Headers.TryGetValue("Authorization", out StringValues assertion);
    var idToken = assertion.ToString().Split(" ")[1];

    var handler = new JwtSecurityTokenHandler();
    var jwtSecurityToken = handler.ReadJwtToken(idToken);

    string tenantId = jwtSecurityToken.Claims.First(claim => claim.Type == "tid").Value;

    GraphServiceClient graphServiceClient = await GetDelegatedGraphServiceClient(idToken, tenantId, new string[] { 
    "https://graph.microsoft.com/ChannelMessage.Send" 
    });

    var chatMessage = new ChatMessage
    {
        Body = new ItemBody
        {
            Content = message
        }
    };

    await graphServiceClient.Teams[teamId].Channels[channelId].Messages.Request().AddAsync(chatMessage);

    return Ok();
}

private async Task<GraphServiceClient> GetDelegatedGraphServiceClient(string idToken, string tenantId, string[] scopes)
{
    string clientId = _configuration.GetSection("AzureAd:ClientId")?.Value;
    string clientSecret = _configuration.GetSection("AzureAd:AppSecret")?.Value;


    IConfidentialClientApplication app = ConfidentialClientApplicationBuilder.Create(clientId)
                                    .WithClientSecret(clientSecret)
                                    .WithAuthority($"https://login.microsoftonline.com/{tenantId}")
                                    .Build();


    UserAssertion assert = new UserAssertion(idToken);
    var responseToken = await app.AcquireTokenOnBehalfOf(scopes, assert).ExecuteAsync();
    string accessToken = responseToken.AccessToken.ToString();

    var graphServiceClient = new GraphServiceClient(new DelegateAuthenticationProvider((requestMessage) =>
    {
        requestMessage
            .Headers
            .Authorization = new AuthenticationHeaderValue("Bearer", accessToken);

        return Task.CompletedTask;
    }));
    return graphServiceClient;
}