An exercise bike with no output for speed or power gives you limited options. A pedal-based power meter like the Garmin Vector series will work, but confirm that the crank arms on the bike will accept a 9/16" pedal axle. Some steel crank arms take a smaller 1/2" pedal axle only.
In addition, a pedal-based power meter will require hard-soled cycling shoes and cleats.
The answers given are absolutely correct but there is another question that I would offer to you.
Why spend the money on power meter pedals when you can use ANY cheap used bike from a garage sale ETC and a used trainer.
A top dumb trainer (Kenetic Road Machine) can be obtained used for $150 and may be used as is with a speed sensor ($20-30) and Z Power or add an In Ride 3 trainer power for $50.
You can use ANY trainer and will have acceptable but variable results as far as accuracy is concerned but it will be fine for general free rides, work outs and social rides and will cost less than power meter pedals.
I recommend anything you get, get the one the broadcasts Blue tooth and Ant +.