Current problem
By default, the official desktop client uses the official URL.
Proposed change
Generate an installation file in a certain way, which should include the custom self-hosted URL. This way, after users download and install it, they can directly enter their account information to log in, eliminating the need for any setup operations. You can refer to SoftEtherVPN
As long as you know the self-hosted URL, you can configure the client by combining with .well-known/fluxer.
Additional information
SoftEtherVPN: https://github.com/SoftEtherVPN/SoftEtherVPN