If you have authorization to make online payments for your client and you have access to your client's bank account information, you should answer "yes" to the question to continue with the bank verification process. You'll need to access your client's bank account to answer the next question on how much we deposited to that account.
User does not have to be a signer on the account; they just need to have access to the test transaction from Bill.com and be authorized to pay bills from the account.