Hi @aufsteigender, welcome. Could you post a link to the app?
I know of https://dictionaries.io/ but this appears to be a different app to the one in the screenshot.
OK, I haven’t actually tested it properly (those dictionaries aren’t cheap!!) but the app installs a macOS Service called “Look Up in Dictionaries” so it should be straightforward:
#popclip
name: Dictionaries
icon: iconify:fluent-mdl2:dictionary
service name: Look Up in Dictionaries