In New York and forward releases, "Agent Chat" is a feature that allows the Virtual agent logo to be added to the entire portal unlike adding the "Virtual agent Service Portal" widget to each page. It enables your end-users to initiate and maintain an Agent Chat conversation on any portal page.
To set a default topic for Virtual agent in "agent chat", there isn't a field to pass the topic parameters but instead needs to be scripted in the 'Server Script' field in Agent chat.
This method is also applicable when setting up IBM Watson as a default topic in the Virtual Agent for a portal.
New York and up using Agent Chat for Virtual Agent.
1)Go to the Topic that you want to set as default topic in sys_cs_topic table.
2)Change the 'Design category' of the topic to 'Setup Topics'
3)Now go to Collaboration -> Virtual Agent -> General Settings
4)In the Greetings topic, change the topic to the required topic. Save the changes.
5)For IBM Watson, use the IBM Watson topic in step 1 from sys_cs_topic.
6)Save the changes and confirm that now directly the default topic opens up on the portal and $sn-va-web-client-app.do
NOTE: This approach will ONLY work if you have only one portal or all your portal share the same default topic.