How to fix a leaking boiler? (leaking from the plastic pipe underneath it)

I’d call a plumber.

duct tape.

Sounds like the "leak" is actually a release through the overpressure pipe. 

structuretech1.com/leaking-relief-valve/

What kind of "boiler" are you talking about?

This?  https://www.powerhouse.com/boiler-sales/new-boilers/hurst/s5x60150

or this? https://www.benfranklinplumbingmn.com/services/water-heater-installation/