Building, integrating, or customizing apps and workflows within Microsoft Teams using developer tools and APIs
To access the activity feed, you will need to use a Bot. Also, you can send notification only for message/card sent in 1:1 chat conversation.
If your bot posts cards/messages into a channel, they'll automatically show up in the user's feed if user has followed that channel.
Sample code to Starting personal conversations
var parameters = new ConversationParameters
{
Members = new ChannelAccount[] { new ChannelAccount(userId) },
ChannelData = new TeamsChannelData
{
Tenant = new TenantInfo(tenantId),
Notification = new NotificationInfo() { Alert = true }
}
};
MicrosoftAppCredentials.TrustServiceUrl(serviceUrl, DateTime.MaxValue);
var connectorClient = new ConnectorClient(new Uri(activity.ServiceUrl));
var response = await connectorClient.Conversations.CreateConversationAsync(parameters);