Text substitution does not work with Slack on Yosemite

Question

I have a text substitution setup within the System Preferences. This works most places within OS X as far as I can tell, but it does not work in Slack. (Their native app for OS X version 1.1.4 - 2814)

Does anyone know why?

UPDATE
It looks like this is not specific to Slack at all

Answer 1

Activate the "Text Replacement" option in the edit menu.

Answer 2

Even after following the instructions in the accepted answer, this still did not work for me until I followed these instructions. Basically, run the following in a terminal:

defaults write -g WebAutomaticTextReplacementEnabled -bool true

or to only change Slack's behavior (and not pollute your global domain):

defaults write com.tinyspeck.slackmacgap WebAutomaticTextReplacementEnabled -bool true

Answer 3

So, it seems that Text Substitutions have not been implemented in Chrome so I assume that is the underlying problem I'm having with Slack.