Hi expert,
I tried hard to use FM – MEGUI_BUILD_REQ_HEADER_PLUGIN which enables custom tab at header level:

then add subscreen in program: SAPLXM02:
Screen number: 1227, added one field for testing purpose.

Create implicit enhancement to method ‘TRANSPORT_FROM_MODEL’ of class LCL_CUST_HEADER_VIEW_MM to view the custom tab at header level.

Result:

Field doesn't display on the screen.
Please kindly advice.
Thanks,
RTN.