I don't believe the T5 built in flash can act as an ETTL master flash with any flash as an ETTL slave.
The T5i built in flash can act as a master flash with the 420EX as an ETTL slave.
Using an external flash that can work as an ETTL master flash, like the 90EX, on your T5 the 420EX could be used as an ETTL slave.
You may be able to use the 420EX as a manual flash with a dumb trigger, but, there are better options for that.
The 420EX will work in the hotshoe of your T5 for a more powerful and tilt swivel ETTL flash on the camera.
