The recommended options for incorporating Named Credentials into your app are described under Credential Management Options. These are:

• Decorate a type string public property with the NamedCredentialAttribute class. This adds a built-in dropdown list of configured credential names.
• Create an app editor for your app and use the CredentialEditorControl Class, an out-of-the-box, reusable control for configuring Named Credentials in an app editor.
• Create an app editor and use the  GetCredentialNames() API to customize a UI for adding Named Credentials.