Visual Studio Installer: Custom Actions

This one took ages to find (much prefer WiX):

Select Custom Actions to view and edit the list.

Unlike WiX you have to code the custom actions (as far as I can tell).

The closest to any documentation I've found is for a very old version of Visual Studio Installer:
Update: The book 'The Definitive Guide to Windows Installer' seems to contain a lot of relevant information for creating this type of installer.