Refresh Tokens

To get a refresh token, you'll need to use the OAuth app authorization flow. It is possible to process this in C#. For the official Dropbox API v2 .NET SDK in C#, you can find an example of getting and using a refresh token in the OauthBasic example (non-PKCE, meant for server-side apps) as well as in the OAuthPKCE example (PKCE, meant for client-side apps).

If this is just for your own use though, that is, you only need to connect your own account, you don't really need to implement that in C#. You can process this manually once and copy/paste the necessary values, which would likely be easier.

For instance, you could:

1. Make your OAuth app authorization URL like this: https://www.dropbox.com/oauth2/authorize?client_id=APPKEYHERE&response_type=code&token_access_type=offline (plug in your app key in place of "APPKEYHERE").
2. Browse to that page in your browser while signed in to your account and click "Allow" to authorize it.
3. Copy the resulting authorization code.
4. Exchange the authorization code for an access token and refresh token like this, e.g., using curl on the command line: (plug in the authorization code from step 3 in place of "AUTHORIZATIONCODEHERE," the app key in place of "APPKEYHERE", and the the app secret in place of "APPSECRETHERE").
   

   curl https://api.dropbox.com/oauth2/token \
       -d code=AUTHORIZATIONCODEHERE \
       -d grant_type=authorization_code \
       -u APPKEYHERE:APPSECRETHERE​

The response will contain a short-lived access token and refresh token that you can then plug in to your code as needed.