Customizing folder icons on Mac Dock

Running Mac 26 I am trying to add an icon to a folder. When I go to the folder and click it, it allows me to add an icon but when I move it to the dock its gone and there is no option to customize a folder that is on the dock. There is an icon on the dock folder for HardDrive. Am I missing something?

[Re-Titled by Moderator]

Original Title: adding icons to folders