This blog will make you clear:
How to register VAG ODIS 4.0 software on VXDIAG A3 VW?
VXDIAG A3 VW ODIS 4.0 Registration instruction
Steps for your reference:
After ODIS installation is complete, right click the ODIS desktop shortcut and choose “Run as administrator”.
You can see the ODIS hardware ID on Registration Dialog, copy this ID.
Open ODIS 4.0.0 Key Generator, enter “1” as the Registration name, paste the Hardware ID then click “Generate”.
You will get the ODIS Registration Key.
Now you can copy the Registration Key to Registration Dialog and click “Register”.
Registration is complete and now you can use VXDIAG A3 ODIS software to diagnose VAG cars!