When adding entries to the activity stream via the ticket conversations widget in the service portal, pressing enter is hard coded to submit the entry, and shift+enter is hard coded to add a new line. which action is performed by enter and shift enter should be a system or user preference - instead of requiring customization of the baseline widget.
1) view any record in the portal in which the ticket conversations widget is displayed. For example, create an incident using the Create Incident catalog item, then view the resulting incident.
2) Type a messagse into the ticket conversations widget
press enter (this will add your entry) or
press shift+enter (this will add a new line to your entry)
This is expected functionality, but, it should be possible to change which action those keypresses perform.
This is expected functionality. The ServiceNow community has several posts which suggest ways to resolve this issue via customization. A few examples are: