There is no coupon code, you simply click it on the product page.
This likely isn't the best option for a portable charger for you unless you're specifically looking for one for the Nintendo Switch, so I understand if you downvote this thread without realizing that, but please keep that in mind if you've read this far. For fellow Switch users, enjoy!
According to https://switchchargers.com/cables/
this portable charger has the highest rating for speed and 2nd highest rating a non-nintendo product can get for safety (none have 5 stars). It has the necessary 15v output via PD to power the dock, which not a lot of these portable chargers have.