You must be a registered user to create a prefix. If you would like to check if a company has a registered prefix, see How to determine if advertiser/client has a registered prefix.
A group and account must be set up in order to create a prefix (see Ad-ID Glossary for descriptions). If the advertiser has never used Ad-ID, you can likely use the set up assistant. When logged in to Ad-ID, access the Create menu in the top navigation bar and select Set Up Assistant. See Using the Set Up Assistant for instructions.
Instructions are also available to Create a Prefix if a group and account already exist.