How do I set up Apple TV without a remote

You cannot. The Apple TV has no buttons so no way for you to interact with it without a remote.

For 4th generation Apple TV or later, the remote can be an iOS device however.

click here ➡️ Set up the Apple TV Remote on your iPhone or iPad - Apple Support

For 4th generation Apple TV or later, the remote can be an iOS device however.

click here ➡️ Set up the Apple TV Remote on your iPhone or iPad - Apple Support

I believe that would work AFTER initial setup with a regular remote.

Haven't tried it, but you may be right. Not sure wether the Apple TV needs to already be set up to provide the pairing code on screen.