You're supposed to follow MetroPCS and the coupon will show up in your newsfeed. It was originally supposed to be for only a few cities but it looks like it's nationwide now.
I used this link (may only work on mobile):
Valid through 2/12. Valid only for in-store purchases. Not combinable with Switcher, Add-A-Line, or New Line Instant Rebates.
Redeem code for an up to $120 instant rebate off of any MetroPCS phone when activating new line of service on that phone. Line must be activated on an Unlimited LTE Plan. Available for new and existing accounts. Code must be presented at time of activation in-store. Not combinable with Switcher, Add-A-Line, or New Line Instant Rebates. Limit 1 per number/2 per account. Not valid on SIM card, accessory, or any other purchases.