Can you open a support case for assistance with this? Your question is a little out of scope for the Developer community and I can’t guarantee you’ll get an answer here.
In case anybody stumbles across this, the documentation is incorrect because it says “externalID”. The property is actually “externalId” which shows up in the attributes list for the directory integration. Remember that this is a property in the directory profile. The expression appuser.externalId will pick up the property to map it into the Okta user profile in the mappings for the directory profile.