This island ID is unrelated to your in-game island: it belongs to a thread on the
Island Journals board!
You can get one by having a thread there, and then looking at the last part of the url that links to it. For example, the link to my (half abandoned) island journal is as you see below, which means that my ID is 558131.
Code:
https://www.belltreeforums.com/threads/the-chronicles-of-narnia.558131/
If filled, the island name under your username becomes clickable, leading people directly to your thread.