You can add comments to a problem directly in the webui, via DAVIS or other tool integrations like jira:
Could I use the webstart client to add it?
If you talk about the webstart client I assume your Question is about Dynatrace AppMon not Dynatrace SaaS/Managed?
You postes in the Dynatrace SaaS/Managed forum, thus my answer only works for that.
For AppMon there is no comment feature for incidents.
Thx so much for your answer