How to change options under Target field on Insert/edit link form which appears on click of a button in translated html field of kb
It is a UI Page 'html_insert_link_dialog': https://<Instance_name>.service-now.com/sys_ui_page.do?sys_id=1a0f41410a0a0b7500b99ae3adf0b605
This selction options are from line 43-49. Update these select option as per your requirement.
Here is more documentation on UI pages if you are new to it: UI pages