WPML asks for a registration key while it is setting itself up. Enter any ten digits, for example 1234567890, and setup completes normally.
That instruction is correct and it has been working for years. Here is why it works, because on its own it looks like it should not.
Why any value gets you through
WPML holds its setup behind a registration step, but the field accepts whatever you type into it. Nothing is checked, so nothing has to be real.
Everywhere else in this knowledgebase we tell you your 99Plugs update key goes in the Update Manager and never inside a plugin. That rule still holds. This field is not connected to us, does not control your updates, and is not a place a real key belongs. It is a door with a lock and no bolt.
If you would rather not keep a made-up number anywhere, put your 99Plugs update key in instead. It works the same and it is one less thing to remember.
Then set up your updates
WPML updates come through the 99Plugs Update Manager, not through WPML’s own updater. See How to enable auto updates.
More on activation prompts across the catalog in My plugin or theme is not activating.