Trying to add a feature service from standalone GIS server 10.7.1 to enterprise portal 10.9.1. There is a built-in user account created in server 10.7.1 to access the secured service.
In enterprise 10.9.1, the option Store credentials with service item. Do not prompt for authentication is missing. Can add the item with built-in user credentials, but asks to login every time the item is accessed. This deployment is not disconnected.
But, the store credential with service option is available when added to enterprise portal in 11.1 (this is a different deployment and network).
Looked at this post: Solved: When does the option "store credentials with item"... - Esri Community, but our deployments are not disconnected.
Also seen this article: Problem: Unable to Store Credentials with a Secured Service Item in Portal for ArcGIS However this is working fine in 11.1 without any SSL import so expect same in 10.9.1 as well.