You can set up a power meter as your power+cadence source and your smart trainer for resistance.
Here's an overview of the process:
Launch the device connection wizard by clicking on the flashing "Connect" in the top center of the main rider screen or when you open a workout
In the top left quadrant, click on "Connect" to initiate the search for devices
Click on your power source to highlight it AND then click "Pair"
In the top right quadrant, click on "Trainer Control" to initiate the search for devices
Click on your smart trainer to highlight it AND then click Pair
Follow the same sequence for Heart Rate
SKIP the External Cadence step in the bottom right quadrant as that will come from your power source
Here's a video of the sequence: