marketing-cloud-app-authorization.html

<script runat="server">
Platform.Load("core", "1");

var client_id = "xxxxx",        //add the client id from the installed package
    client_secret = "xxxxxx",    //add the client secret from the installed package
    subdomain = "xxxxx",        //add the 28 character subdomain (starts with mc....xxxx)
    redirect_uri = "xxxxx";     //add the url of the CloudPage that hosts your app

var auth = true;

var authToken = Platform.Request.GetCookieValue("authToken");
if (authToken == null) {
    var authCode = Request.GetQueryStringParameter("code");
    if (authCode == null) {
        Platform.Response.Redirect("https://" + subdomain + ".auth.marketingcloudapis.com/v2/authorize?response_type=code&client_id=" + client_id + "&redirect_uri=" + redirect_uri);
    } else {
        var url = "https://" + subdomain + ".auth.marketingcloudapis.com/v2/token"
        var contentType = "application/json";
        var payload = {
            "grant_type": "authorization_code",
            "code": authCode,
            "client_id": client_id,
            "client_secret": client_secret,
            "redirect_uri": redirect_uri
        };
        try {
            var accessTokenRequest = HTTP.Post(url, contentType, Stringify(payload));

            if (accessTokenRequest.StatusCode == 200) {
                var tokenResponse = Platform.Function.ParseJSON(accessTokenRequest.Response[0]);
                var accessToken = tokenResponse.accessToken;
                Platform.Response.SetCookie("authToken", accessToken);
                auth = true;
            }

        } catch (error) {
            auth = false;
        }
    }
}
</script>