Need an idea to create an integration app in microsoft teams.

You can achieve (almost) this using either Incoming Webhooks (https://docs.microsoft.com/en-us/microsoftteams/platform/webhooks-and-connectors/how-to/add-incoming-webhook) or with Office 365 Connectors (https://docs.microsoft.com/en-us/microsoftteams/platform/webhooks-and-connectors/how-to/connectors-creating). With this you can send a card to a Teams Channel.

However, you can't send a notification to specific users with this. If that's a hard requirement then you need to implement a bot that sends proactive messages: https://docs.microsoft.com/en-us/microsoftteams/platform/bots/how-to/conversations/send-proactive-messages?tabs=dotnet