Yes, you can likely select Maths as a domain in CUET UG even though you were a PCB student in your main Class 12. Here's why:

Delhi University's rules for CUET UG say you need to appear in CUET in subjects that you have passed in Class 12. Since you appeared privately and passed Additional Mathematics, DU should consider this as a subject you've passed.

Therefore, when you fill the CUET UG form, you should be able to choose Mathematics as one of your domain subjects alongside your language paper.

However, its super important to check the specific eligibility criteria for the exact course you want to join in DU. Different courses might have specific subject requirements in CUET. For example, a B.Sc. Maths (Hons) will definitely need Maths in CUET.

So, while you can probably choose Maths in CUET because you passed it as an additional subject, always double - check the DU course  requirements on their official admission website to make sure your subject combination in CUET will make you eligible for the course you're aiming for.

I hope this helps.