How to post a message to Slack using Slack App + OAuth2

✅ Posting Method Using an Access Token (Slack Web API)

Slack’s “OAuth Token (such as xoxb-…)” is a key used to post using Slack’s Web API. In this case, unlike Webhooks, you POST to the Slack API Endpoint with an Authorization: Bearer header.

🔑 Required Prerequisites

You need to include chat:write in the OAuth Scopes of your Slack app:

Setup Steps

Go to https://api.slack.com/apps
Create an app or select an existing one
Under “OAuth & Permissions” > Scopes → Add chat:write
“Install to Workspace” or “Reinstall” to get the Access Token (e.g., xoxb-xxxxxxxxxx)

💻 C++ Code (Posting to Slack API using WinHTTP)

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#include <windows.h>
#include <winhttp.h>
#include <iostream>

#pragma comment(lib, "winhttp.lib")

bool PostSlackMessage(const std::wstring& accessToken, const std::string& channel, const std::string& text) {
    const wchar_t* host = L"slack.com";
    const wchar_t* path = L"/api/chat.postMessage";

    HINTERNET hSession = WinHttpOpen(L"SlackPoster/1.0",
                                     WINHTTP_ACCESS_TYPE_DEFAULT_PROXY,
                                     WINHTTP_NO_PROXY_NAME,
                                     WINHTTP_NO_PROXY_BYPASS, 0);
    if (!hSession) return false;

    HINTERNET hConnect = WinHttpConnect(hSession, host, INTERNET_DEFAULT_HTTPS_PORT, 0);
    if (!hConnect) return false;

    HINTERNET hRequest = WinHttpOpenRequest(hConnect, L"POST", path,
                                            NULL, WINHTTP_NO_REFERER,
                                            WINHTTP_DEFAULT_ACCEPT_TYPES,
                                            WINHTTP_FLAG_SECURE);

    // Authorization header and Content-Type
    std::wstring headers = L"Content-Type: application/json\r\n";
    headers += L"Authorization: Bearer " + accessToken + L"\r\n";

    // JSON body
    std::string body = R"({"channel":")" + channel + R"(","text":")" + text + R"("})";

    BOOL result = WinHttpSendRequest(hRequest,
                                     headers.c_str(),
                                     (DWORD)-1,
                                     (LPVOID)body.c_str(),
                                     body.length(),
                                     body.length(),
                                     0);

    if (!result || !WinHttpReceiveResponse(hRequest, NULL)) {
        std::cerr << "Send error\n";
        return false;
    }

    // Check status code
    DWORD statusCode = 0;
    DWORD size = sizeof(statusCode);
    WinHttpQueryHeaders(hRequest,
                        WINHTTP_QUERY_STATUS_CODE | WINHTTP_QUERY_FLAG_NUMBER,
                        WINHTTP_HEADER_NAME_BY_INDEX,
                        &statusCode, &size, WINHTTP_NO_HEADER_INDEX);

    WinHttpCloseHandle(hRequest);
    WinHttpCloseHandle(hConnect);
    WinHttpCloseHandle(hSession);

    return (statusCode == 200);
}

int main() {
    std::wstring token = L"xoxb-your-access-token"; // Access Token
    std::string channel = "channel-ID-or-#general";       // Example: "#general" or "C0123456789"
    std::string message = "I tried posting to Slack from C++!";

    if (PostSlackMessage(token, channel, message)) {
        std::cout << "Post successful!\n";
    } else {
        std::cerr << "Post failed.\n";
    }

    return 0;
}

📌 How to Get the Channel ID

Using just the channel name (e.g., #general) may sometimes result in an error. A more reliable method is to use the ID found in the URL of the Slack channel page.

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https://app.slack.com/client/Txxxxx/C0123456789
                                 ↑ This part is the Channel ID

✅ About Token Types (Supplementary)

Token Format	Purpose	Example Use Case
`xoxb-...`	Bot Token (Recommended)	Post, edit, delete
`xoxp-...`	User Token (Not Recommended)	Non-bot operations
Refresh Token	Long-term token renewal	Usually not used

📝 Summary

To post to Slack from C++, the Slack Web API + Bearer Token method is reliable.
Use the chat.postMessage endpoint with HTTPS POST.
The token must be sent in the Bearer header.
Specifying the channel by ID is recommended.