To add a new application to a channel of a Team, go to the channel, and click the + on the tab bar.
In the pop-up window, click the "Application".
To create a new application, in the next page, select the workspace from the left workspace list. At the buttom of the right appliations list, click "+ New Application". And from the pop-up menu, select to create from scartch of with a template.
If create a blank application, click "Start from scratch", then fill in the app name, click "Enter" to create the app.
If create from the template, in the next page, select the template, and click the "Install" button to create a new applicaiton.
After the application is created, you can select this application, and click the "Save" button to add your applicaiton to the teams tab.
After the application is added to the tab, you can design your application per your needs.