You do not need to fill any website name, it s an option. Please read last link below, the image from that link.
Go to Builder Profile Page
If you want to show your name on your GPT switch on the button in same line Name. If you do not want to show your name switch it off.
Also you can read these OpenAI Help pages:
gpt-builder
creating-a-gpt
knowledge-in-gpts
building-and-publishing-a-gpt
