The serviceUrl is channel specific, not user, bot, or team. So, for example, Direct Line would be "directline.botframework.com"; Slack would be "slack.botframework.com"; and, so on. You can reference a small bit of information here.
As for sending proactive messages, I would recommend you read over the BotFramework docs on the topic, the Teams docs, as well as review the samples available in the BotBuilder-Samples repo under Advance bots (#16).