How to get the free band for the watch?

How? The instructions using that link state "Between 8/10/18 and 9/8/18, pre-order or purchase (at full retail price), from a participating Best Buy, Amazon, or Costco, or between 8/24/18 and 9/8/18, pre-order or purchase (at full retail price), from a participating Target or Walmart, a Samsung Galaxy Watch (SM-R800NZSAXAR (46mm), SM-R810NZKAXAR (42mm Black), SM-R810NZDAXAR (42mm Rose Gold)".
 
How? The instructions using that link state "Between 8/10/18 and 9/8/18, pre-order or purchase (at full retail price), from a participating Best Buy, Amazon, or Costco, or between 8/24/18 and 9/8/18, pre-order or purchase (at full retail price), from a participating Target or Walmart, a Samsung Galaxy Watch (SM-R800NZSAXAR (46mm), SM-R810NZKAXAR (42mm Black), SM-R810NZDAXAR (42mm Rose Gold)".

Try it, the worse could happen is they say no.
 
Don't think it's possible for the tmobile one because it has a different model number than the ones listed on the promo page.