Configuring Fiddler to use company network's proxy?

I'm trying to get Fiddler to work with my company's proxy. Every external request is returning 407.

So far I've tried adding oSession.oRequest["Proxy-Authorization"] = "YOURCREDENTIALS"; to the customized rules where I used my USERNAME:PASSWORD in base64. Still no luck.

Any ideas? Thanks.

Note: There is an answer with a higher voting available. Because of SO sorting it is below the accepted answer.

I had the same problem, too, and solved it like this:

1. Started Fiddler with it's standard configuration.
2. Started IE and made a HTTP-request to an external web-site.
3. The proxy authorization dialogue popped up, where I entered my credentials.
4. In Fiddler searched the request headers for "Proxy-Authorization".
5. Copied the header value which looked like "Basic sOMeBASE64eNCODEdSTRING=" to the clipboard.

6. Altered the CustomRules.js with the following line within OnBeforeRequest:

   oSession.oRequest["Proxy-Authorization"] = "Basic sOMeBASE64eNCODEdSTRING=";

So my approach was quite similar to yours just that in advance I checked what kind of proxy authorization the server required by using Fiddler to debug the authorization header. That way I found out I had to add "Basic" before the Base64 encoded credentials and I didn't even have to use the tool to encode the credentials to Base64. Just copied the value from the proxy authorization header.