You cannot choose the Accredited Host (AH) platform you use for the verification. A facility chooses an AH platform to upload their assessment data: you will need to use the same AH for the verification.
As a Verifier, you will receive an automated notification via e-mail (from the Accredited Host platform) when your Verifier Body has assigned you to a verification on that Accredited Host. In the automated e-mail, you will be asked to create an account on the Accredited Host platform or log in to your existing account.