The specified page cannot be customized. Only reports found in the Reports tab, reports with an Edit Report button on the left, can be customized. You can build a custom report that lists the information you are after.
This report recreates the Assets page and can be added to your Lansweeper installation by following
these instructions. To add last logged on users to the report, tick the Username and Userdomain fields in the tblAssets table in the upper section of the report builder.