I was able to do this tonight though the iPhone app. Booked a KLM reward redemption and logged into the China Airlines app using the ticket number and my name (first and middle squished together.)

Once you’re in the flight information, choose “passenger” at the very top. From there you’ll see “seat” with a pencil icon to click.

“Manage booking” did not work for me.

Good luck!

Also - does anyone know if the first row of business on the 777 has a larger footwell? That’s what I came to FT to try to find out. Flying SFO-TPE.