Add banner motd to postgres database
You cannot force that on the server side, because the server cannot control what a client displays.

But yan can do it on the client side: create a .psqlrc file in your home directory that contains something like:

\echo Seien Sie bitte vorsichtig.....


But I assume, given the text of the warning, that you want this bannet only for certain sensitive databases.

For that, you could have  banner table in every database, and the statement in .psqlrc would be like:

\set QUIET on
\pset format unaligned
SELECT * FROM banner;
\pset format aligned
\set QUIET off