Discord: Channel > Edit > Integrations > Webhooks > New Webhook > Copy URL. Slack: an Incoming Webhook URL. Or any HTTPS endpoint that accepts JSON.
Telegram: create a bot with @BotFather, start a chat with it (or add it to your group/channel), then use https://api.telegram.org/bot<token>/sendMessage?chat_id=<your chat id>. Get your chat id from @userinfobot.
