A quick search over the internet has shown that Electrs uses "banner":
The server I use
(electrs), just provides a "Banner" option that is just free text that can be specified at runtime or in a config file that is the message shown in the Electrum console and you can add a donation address in that message.
and it should be configurable even as command line flag (you can search for banner in
release notes).
Maybe it's a start.