The offer is not for a free standard HDMI cord, but for a free short HDMI extension cord for owners of Roku Streaming Stick products, and I think it is limited to owners in the US..
Roku Streaming Stick models are equipped to plug directly into the HDMI port on the TV -- no connecting HDMI cable is required.
In the event your TV is not physically configured to allow this, you may use an HDMI extension cord to move your Streaming Stick to a more convenient location. This is also recommended in cases where signal leakage from the TV interferes with wifi reception by a Streaming Stick that near to the TV. Roku offers a free short HDMI extension you can request here: https://my.roku.com/hdmi. (This offer is for owners of Roku "Streaming Stick" models only, verified by the serial number of your unit.)
There's nothing special about the Roku HDMI extension, it's just a short (4 inches or so) HDMI cord with an HDMI plug at one end and an HDMI socket at the other. I use one several feet long to locate my Streaming Stick in an equipment closet on the other side of the wall from my TV. You may be able to find an HDMI extension locally and not have to wait for someone to ship it to you.
Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T