Adding a Subsite to the Quick Launch (Left Menu) in SharePoint
Before You Begin
You will need to get the URL of the SharePoint item you wish to add. To get it, go to Site Contents > Click on the item icon or subsite > copy the URL of this item.
To add a page, subsite, list, library or external link to the left menu (Quick Launch) in your SharePoint site, click EDIT LINKS

Then + Link

- Now click on EDIT LINKS and paste in the URL.
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