As explained in the solution to this question, to enable autocompletion in Basic macro editor you should enable “experimental features”, which are marked as “unstable”.
Please correct me if I’m wrong, (the thread was authored in 2015) but as of today it still appears to be relevant?
Perhaps there has been some other way to set up IDE for libreoffice Basic macros?
I found instructions how to set up IDE in the official documentation here, but this seems to focus on C/C++ and not Basic.