You can create a Zapier flow to send messages to Slack when a campaign is completed in Instantly.
Step 1: Copy Your Zapier API Key from Instantly
In the Classic Frontend
Go to Integrations -> Apps
Copy Zapier API Key
In the New Frontend
Go to the Integrations tab.
Use the search bar in the Apps section to find the Zapier integration
Click "Copy API key"
Step 2: Configure Instantly Event
In your Zapier account, click on Create new Zap, select Instantly and New Event
Select Instantly account or connect it by pasting the Zapier API key.
In the Configure tab, select the campaign and for event type, choose 'Campaign Completed'.
Test your trigger.
Step 3: Configure Slack Action
Then click + to add Slack and choose Event: Send Channel Message.
Sign in to your Slack account to connect it with Zapier.
Choose your designated channel, customize the message and the bot.
Test your action and see the data come through. (If the test fails, make sure you have a completed campaign.)
Now you can publish your Zap and receive a Slack message each time a campaign is completed in Instantly.