how to create a keyboard shortcut for a toolbar click in Mac osx high sierra

And as long as we are using an anonymous application, and right and left indent keyboard shortcuts are likely implemented differently in each application — select the text, and then try ⌘+] for right indent, and ⌘+[ for left indent. These work in the Sublime Text 3, and Textastic programmer's editors, or Pages v10.1 to name a few applications.

You cannot create keyboard shortcuts to toolbar items, only to unassigned application menu items where there is no conflict with the keyboard shortcut.

Which toolbar click?

Well, you don't say what App you are using, but in Word®...

To increase indent of the selected paragraph, press Ctrl + M. To decrease indent, press Ctrl + Shift + M.

Also see...

https://superuser.com/questions/460052/is-there-a-keyboard-shortcut-to-indent-a-nested-bullet-point-in-a-table-cell-the