For sensors it is possible to define rules, which trigger notifications, state changes and tasks (see the article 'How are Rules used'). To enable forecasting for a sensor, you need to setup another rule.
Start by opening the rules overview page, which can be accessed via the machine menu. To add a new forecasting rule, click the 'add rule' button:
A pop-up will open, where you can set up a condition rule or forecasting rule. Choose the forecasting rule:
Then you select the sensor for which you want forecasting enabled, and set the value with its condition and probability. Also if you want to be notified and/or which task (see the article 'What are machine tasks?') should be triggered: