Register a Secure Service with ArcGIS Online
This article describes how you can register a secure service with ArcGIS Online so you can access the service in UtiliSync.
UtiliSync does not directly support secure services published from ArcGIS Server where users are required to sign in each time the service is loaded. However, in some cases, it is possible to "register", or add, the service to ArcGIS Online. See this ArcGIS article for details. Note, in step 5 it mentions secure services. Be sure to select the option to store the credentials with the service. If you do not see that option, see the ArcGIS article below.