Develop a messaging extension

Run ngrok.exe http 5000, because our web api project will listen on port 5000.
Copy the ngrok url.
Create a bot using Teams App Stdio or the BotFramework web https://dev.botframework.com/bots/new. Greater detail can be found here. Regards to the Messaging endpoint, input the ngrok url with the postfix /api/extension

When a bot is created, remember the Microsoft App ID which will be used in the next step.

Run dotnet new teamsmsgext --name SampleApp to generate the project.
Open SampleApp/manifest/manifest.json file, paste Microsoft App ID you got in previous step.

Zip SampleApp/manifest folder into manifest.zip file.

Upload manifest.zip file into your Teams. Manage Team -> Apps, click the bottom-right link Upload a custom app

Start the project by running dotnet run.
All done, you can switch to Microsoft Teams, go to a team's channel into which you have uploaded the custom app(the zip file), click the ... button, you can find your messaging extension.