The dunst project has a command line utility called dunstctl but no bash
completion yet.
Drop the dunstctl file into the folder for bash completions on your machine, usually found in these places:
/usr/share/bash-completion/bash_completion/completions//etc/bash_completion.d/~/.bash_completion.d/(might not be loaded on your setup)
I'm planning to submit this as PR during hacktoberfest to the dunst project, so please be cool and don't steal that PR :)