Hello Nekenpongen,
Thanks for your information.
We have finished the test and finally get it.
Please see the outcome:

If this is the case, you need to create a new Inbound sync rule to use the
Custom attribute to get the
employeeID's value, please see:

To create a new Inbound sync rule, please check
here to get the detailed reference. And I will send you a word file with the detailed steps in a private message.
If this isn't the case, as a workaround, you can add the employeeID to the Notes esction in your local AD and this attribute will be synced to Office 365 EAC, please see:

Regards,
Rudy