Explanation

Step 1: Create a dedicated consumer group..
Add a consumer group to your IoT hub.
Applications use consumer groups to pull data from Azure IoT Hub. To reliably read data from your IoT hub, provide a dedicated consumer group that's used only by this Time Series Insights environment.
Step 2: Add a new Time Series Insights event source.
Add a new event source
* Sign in to the Azure portal.
* In the left menu, select All resources. Select your Time Series Insights environment.
* Under Settings, select Event Sources, and then select Add.
* In the New event source pane, for Event source name, enter a name that's unique to this Time Series Insights environment. For example, enter event-stream.
Step 3: Configure the Time Series event source to connect to an existing IOT hub Step 4: For Source, select IoT Hub.
Step 5: Select a value for Import option:
If you already have an IoT hub in one of your subscriptions, select Use IoT Hub from available subscriptions.
This option is the easiest approach.

Reference:
https://docs.microsoft.com/en-us/azure/time-series-insights/time-series-insights-how-to-add-an-event-source-ioth