This guide is only applicable to Users who access the LMS via Workspace. If you don't use Workspace, please see the guide Set/Add a Line Manager for assigning a line manager directly in the LMS.
If you access the LMS, you'll only be able to assign a Line Manager reporting line via Workspace Members. However, other manager relationships and delegate managers are assigned under the reporting lines tab of a user's profile within the LMS. To assign other manager relationships see the guide Set Reporting Lines and Relationships.
Once logged in to Workspace, click on 
to go to the Members section.
This will open up the Members tab and will display all of your Users.
Search for the User you want to assign a manager to using the search field.
Click the 3-dot menu in the top right corner of the User's card and click Assign Manager.
Search for the Manager in the Name field. Click on the name of the User you want to assign as Manager when they show in the area below the Search box.
The name will turn green and will display a tick to show they are selected. Click on Assign.
Click Apply on the confirmation window.
You may briefly see a progress dial while the Manager is assigned to the User.
Once assigned a confirmation window will appear. Click on Close.
The assignment will be reflected in the LMS within the hour after setting in Workspace.