From your "Accountant Profile" you can mask/white-label TPP's domain name with your own subdomain. The CNAME record will point your subdomain to the IP address of the destination app.taxplannerpro.com.
In order to set up your custom domain you need to add a CNAME DNS record pointing to app.taxplannerpro.com.
CNAME- A Canonical Name record creates an alias for a fully qualified domain name. When a user attempts to access a domain name that is actually an alias, the DNS system substitutes the real domain name — known as the Canonical Name — for the alias.
DNS- The domain name system is responsible for translating domain names into a specific IP address so that the initiating client can load the requested Internet resources.
There are a couple tutorials explaining this linked in this article as well as under the "User Domain" section as highlighted below.
Please, refer to the resources below on how to set this up with common DNS providers.
Once you have added your "Domain Name" in the "Accountant Profile" be sure to click "Update" for it to take affect.