Click View against the asset you want to set up/edit the SMS notification for in Registers>Assets.
Scroll all the way down to RESPONSIBLE STAFF. Select the Responsible Staff from the drop-down menu, enter an SMS time you would like them to receive the weekly pre-start SMS reminder, toggle on the Pre-start SMS then scroll down and click Submit.
After you click Submit, the following pop-up will appear.
You have now assigned this asset to a permanent responsible staff member. If you've specified an SMS time, they'll now receive a weekly pre-start SMS reminder at that time every Monday.