If your professional uses GlossGenius, they have a direct link to their page. They've likely sent you an email or a text message with that link, so look there first! If not, you may need to call them in order to get the address.
As an alternative, you could search Google to find their booking site.
For example, if you want to book with John Doe and you know he uses GlossGenius but you don't know his full URL, go to www.google.com and type in John Doe GlossGenius. This will almost always take you to the correct site, but check their location address to make sure you have the right John Doe!