Hi Owen,
For configuring login screen for launchpad and launchpad designer, please do following:
1. Login to the SAP Gateway system.
2. Access the activity using the following navigation options:
Transaction Code SICF
3. Choose Execute. The Maintain Service window displays.
4. Double - click on the node /sap/bc/ui2/flp
5. Choose Display/Change button.
6. Choose the Error Pages Tab.
7. In the Logon Errors tab. Choose System Logon and then choose the Configuration button beside.
8. On the System Logon Configuration dialog box, choose Define Service-Specific Settings.
9. Choose Custom Implementation and input /UI2/CL_SRA_LOGIN into the ABAP Class text field.
10. Choose Save as Global Settings.
11. On the Create/Change a Service screen, choose Save.
12. Repeat above steps for Launchpad Designer service /sap/bc/ui5_ui5/sap/arsrvc_upb_admn.
Result
The default logon page is configured. Then you will see this logon page if you visit the Launchpad through http(s)://<FronEndServer>:<Port>/sap/bc/ui2/flp
Regards,
Ning