Tekst die op het eerste bericht van een persoon moet verschijnen

[ondrej]

@vwcnot

which is added to the bottom of the first post

Closest thing I can think of is a staff notice. Only thing is, is that it appears at the top instead of the bottom of a topic.

Post notices (staff notices)

In Discourse 2.3.0.beta4 we added a special call-out notice for new/returning users. Staff can now add custom call-outs above posts, which appear similar to the new user first post call-out. In latest Discourse, we made it so you can also use Markdown. [49] This notes can be added from the button (post admin actions) and Add Staff Notice.