If you have a Genius account, and you want to use it when you're working in the Travel Agent Tool, please follow the steps below.
1. Login the Travel Agent Tool
Login page: https://admin.booking.com/agent/
Login name is your email address (the one you use to register for the travel agent programme)
After filling all filters on the Dashboard and clicking "Search", you will be directed to the Booking.com search results page.
2. Log into your Genius account:
On the upper right corner of this page, click "Sign in" button and log in with your Genius account credentials.
3. If you are making reservations using the link (https://www.booking.com/index.html?aid=****, where **** stands for your Affiliate ID), simply log into your Genius account in the upper right corner.