In this case, if they choose Bank transfer, they can book instantly. You need to setup online payments to set this option.
If you would like more control over bank transfer payments, we recommend to set the service to Manual appointment confirmation.
Step-by-step guide:
Go to Payments.
Click the Settings tab, and click the Payment Settings button.
Scroll down to Additional payment methods section.
Under Bank transfer, click on Enabled toggle.
Save when you're done.
We recommend you to add your bank transfer/payment link details to an email template.