Login to your AppBeat account (or create new one if you don't have it yet).
Click "Add new periodic monitor" and select "SSL Certificate" as shown on screenshot below:
Click "Next" button and enter your URL:
Click "Next" and close wizard with "Finish".
Your certificate will now be periodically monitored and when it is about to expire (30 days or less - you can change this in check settings), you will automatically receive notification (we support email, SMS, Slack, web hooks, ...).
Simple as that!