Use a shared folder with Time Machine on Mac
You can use a Mac on your network as a Time Machine backup destination by setting up a shared folder. After you set up the shared folder, other Mac computers on your network can use it as a backup destination.
On your Mac, choose Apple menu > System Preferences, then click Sharing.
Select the File Sharing tickbox.
Click Options, make sure “Share files and folders using SMB” is selected, then click Done.
Click the Add button at the bottom of the Shared Folders list.
Navigate to and select the folder you want to use for sharing, then click Add.
Control-click the name of the folder, then choose Advanced Options.
If available, click the “Share over” pop-up menu, then choose SMB.
Select “Share as a Time Machine backup destination”.
If desired, select “Limit backups to”, then enter a size.