Click on the middle tab and select your seamless tile as your pattern. Flood fill the background strip with your pattern. It should look something like below. Magic Wand and click in the octagon border, hold the shift button down and keeping clicking till the whole border is selected, it should look something like below. Click on the Flood Fill tool again and use the same settings as before except change the opacity to 100 as below.
Flood fill the selection.
It should look something like below. You should now have something like below. Go to Layers, Merge all flatten. You can add a tube or something to the border if you want. Sign and Save.