March 31, 2020 10:07 EDT AM
Command-F1 should cause your iMac's display to mirror on the external display (which in this case would be your TV).
But just plugging in an external display should give you a 2nd monitor which should so something.
But if your iMac is just giving you a white screen, there may be more happening.
You might try the following:
try an SMC reset and an NVRAM reset
Reset the System Management Controller (SMC) on your Mac - Apple Support
How to reset NVRAM on your Mac - Apple Support
Try booting into Safe mode
This will not load any 3rd party additions, it will load some more conservative Apple drivers (may cause screen flicker), and it will clear some kernel caches (a cache is saved data in a form that can speed up a program, but is totally redundant to the original source, and thus can be safely cleared). Booting into Safe mode is just an experiment, but can frequently eliminate any 3rd party interference, or a cached item out-of-sync with the world. (Verify Safe mode via Applications -> Utilities -> System Information -> Software -> Boot Mode -> Safe vs Normal)