How do I attach a web branding to my onboarding type?

Firstly, enter 'Onboarding types' in the 'Prospect Onboarding' drop down:

from here, select the onboarding type the web branding will be relevant to.

When editing the onboarding type, you will see the option for 'web branding'

It is here that you can select one of your web branding templates to be used within this onboarding type.