1. Add reference to “System.Configuration.dll” file

2. Get Web application name.

3. To get current SharePoint site Web application name, use SPContext.Current.Site.WebApplicaiton.Name

4. Get Configuration object from WebConfigurationManager class

5. Get AppSettings object from Configuration object.

6. Get value

Code:
             string WebApplicationName = string.Empty;
             Configuration Config;
              AppSettingsSection AppSettings;  
             
             WebApplicationName = SPContext.Current.Site.WebApplication.Name;   
             Config = WebConfigurationManager.OpenWebConfiguration(“/”, WebApplicationName);
             AppSettings = Config.AppSettings;

             if (AppSettings != null)
             {
                        if (!string.IsNullOrEmpty(AppSettings.Settings[“Key”].Value))
                        {
                           // write code
                        }

              }