From your link expand the Share a Screen option.
Select the link in the sentence:-
For information about using the Screen Sharing app, see Screen sharing overview.
That is here Screen sharing overview - Apple Support
At the bottom select Set up and use screen sharing
(plain text like above https://support.apple.com/en-gb/guide/mac-help/set-up-and-use-screen-sharing-mh1 1848)
Expand the first item to read:-
- Open Sharing preferences (choose Apple menu > System Preferences, then click Sharing).
- Select the Screen Sharing checkbox.
If Remote Management is selected, you must deselect it before you can select Screen Sharing.
- To specify who can share your screen, select one of the following:
All users: Any of your computer’s users can share your screen, except sharing-only users and guest users. If your Mac is connected to a network account server, anyone with an account on that server can share your screen using their network user name and password. For more information, see Join your Mac to a network account server.
Only these users: Screen sharing is restricted to specific users.
- If you selected “Only these users,” click the Add button at the bottom of the users list, then do one of the following:
- Select a user from Users & Groups, which includes all the users of your Mac except sharing-only users and guest users.
- Select a user from Network Users or Network Groups, which includes users and groups with network server accounts. Network users and members of network groups can use their network name and password to connect to your Mac for screen sharing.
- To set additional options for screen sharing, click Computer Settings, then select one or both of the following:
Anyone may request permission to control screen: Before other computer users begin screen sharing your Mac, they can ask for permission instead of entering a user name and password.
VNC viewers may control screen with password: Other users can share your screen using a VNC viewer app—on iPad or a Windows PC, for example—by entering the password you specify here. If you select this option, you should create a very secure password.
As in earlier versions of OS X and the Messages that first had Screen Sharing started from iMessages rather than a Buddy List (which was in-app from iChat days) it was needed.
If from your Link the person chooses to expand the "If you can't Share a Screen" then it goes to the "Set up and use screen sharing" page anyway.
Please check all the links in the information you supply because it it is not always a case of looking for the answer you think you want but the one that will help.
This iChat 4 page has a summary at the bottom about what was added in each version including iChat 4 which is when in-app Screen Sharing started.
It is one of about 360 pages on iChat.
There were 6 versions of iChat
Messages continues the number starting at Version 7
There were two version 8's in Mavericks and Yosemite (although both are different)
High Sierra calls it version Version 11.
Subset numbers that were common in iChat and early Messages have disappeared. (it makes it difficult to tell when Messages has been updated during the "life" of an OS, although Messages 7 does have two documented versions 7.0 and 7.0.1 that came out in the Mac OS X 10.8.2 update and is why the Serial Number is needed now to login in a Device Specific manner.
Hope that helps.
8:14 pm Wednesday; December 6, 2017
iMac 2.5Ghz i5 2011 (Sierra)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone 6 iOS 11.x and an iPad (2)