So per the API docs here you can do a task like the local executed shell script below.
Optionally you can send a body payload to configure a name/description as well
URL='<%=morpheus.applianceUrl%>'
INSTANCE='<%=instance.id%>'
TOKEN='<%=morpheus.apiAccessToken%>'
curl -X PUT "$URL/api/instances/$INSTANCE/snapshot" \
-H "Authorization: BEARER $TOKEN" \
-H "Content-Type: application/json"