How to: Add an application configuration file to a C# project

To add an application configuration file to a C# project

1. On the menu bar, choose Project > Add New Item. The Add New Item dialog box appears.

2. Expand Installed > Visual C# Items, and then choose the Application Configuration File template. 

3. In the Name text box, enter a name, and then choose the Add button.

4. A file named app.config is added to your project.

Rujukan:

How to: Add an application configuration file to a C# project - https://docs.microsoft.com/en-us/visualstudio/ide/how-to-add-app-config-file?view=vs-2019