Air Conditioning - How to clean the Drain pipe ?

From reading the posts in this forum, it appears that I have to clean the pipe leading out of the drain pipe to solve the issue I have. However the pipe's beginning/end is under a big aluminium duct and is in an awkward/unreachable position to access it.

How do I drain a cup of bleach through it ?
Its horizontal leading out of the drain pan for about 3 feet, and then goes down (vertical). I am not sure if I need to unscrew the pipe at the end where it begins/fits into the drain pan.

Thanks in advance ...

The bleach will not clear a blockage. It is only for prevention. If the pipe is blocked, use a shop vac on the discharge end. Make sure to cap any vent tees while cleaning then open them back up when done.