Originally Posted by TemboOne

I know nothing about Google Flights but your question prompted me to check it out.

Just did a dummy booking (YSB-YEG) on Google Flights and when the purchase option appeared I clicked "Buy with Air Canada".

Then wthin AC there is a button to "Modify your search".

Clicking that button brings up a window with a number of options, the last of which is a link to open a window where you can input or remove a promo code. I did this and it works, amending the original Google Flights reservation within AC and reducing the base fare by 20%.

Please note this is just a first time visitor to GF so there may be better easier ways to do this!