Frappe doctype LInk field not loading values

I am following frappe tutorial https://frappe.io/docs/user/en/tutorial/naming-and-linking , I followed exactly same but library members first and last names are not updating automatically while creating a membership. How to resolve this?

Field type for Librabry Member should be Link.

Pls check.

So I discovered using erpnext app that sytnax should be library_member:first_name i.e. it should be a colon not a dot. Why frappe tutorials are not updated?