Odoo made the timesheet portal feature to let the customers of any company track the progress of tasks spent time so it's against what Odoo intended to do. So you won't find it in Odoo. 

However there is an alpha version that Odoo Community (OCA) have build for Odoo 12 community and it's not that perfect but it will do the tick check the module here 'hr_timesheet_portal'. and you can try to upgrade the module or request the upgrade by opening a ticket in github.

Happy to help :) an upvote will be awesome