Explanation in the comments

Be a heavy

Then, press a key that has the "Dispenser here" voiceline

Start a taunt with a voiceline (say, sandvich), play that "dispenser here" line when he's about to start eating

Boom! He said "Pootis"!

You can also hurt yourself as soon as he said Pootis, but it only works on certain locations of the map

and one thing: requires proper timing