To add the Save button to the form header, users need to turn on the system property glide.ui.save_button, which enables the Save button on all forms1. Users can navigate to All > System Properties > UI Properties and search for the property name, then set the value to true1. Alternatively, users can use the sys_properties.
list URL suffix and filter by the property name1.
References
* How to add or enable Save Button on all the forms across a Servicenow Instance - Support and Troubleshooting - Now Support Portal