The easiest way to do this is through the Microsoft.Bot.Builder.Teams middleware.
Related Thread - How can my Teams bot start a new 1:1 chat with a known user
Start a one-on-one chat: https://docs.microsoft.com/en-us/microsoftteams/platform/resources/bot-v3/bot-conversations/bots-conv-personal
To send proactive messages: https://docs.microsoft.com/en-us/microsoftteams/platform/bots/how-to/conversations/send-proactive-messages?tabs=dotnet