Unfortunately, if you are not using a tool that automatically provides the correct header information values for you, you are left to look up those appropriate values yourself since Fiddler does not provide any default header information.
If you want to generate your Bearer Token, you can set up Fiddler to pass the following parameters in the Request Body like so:
Once you have obtained the Bearer Token, in the Composer Headers section, you type in the following:
Authorization: Bearer <bearer token>
Therefore, your Fiddler Composer screen will look something like this:
That is all there is to it!!