Add external volume app folder to Launchpad

Question

I have a primary volume and secondary volume which I store apps on in a folder called HDD Applications. Launchpad only looks at the /Applications folder on the primary drive for apps. How can I get Launchpad to also use the HDD Applications on the other drive when showing apps?

I have tried symlinking the directories with ln -s "/Volumes/Macintosh HDD/HDD Applications/" "/Applications/HDD Applications" but that does not work and would like to not have to symlink each individual app.