1. In Fiddler’s Composer tab, Select the "Parsed" tab.
2. In the same tab, Change GET to POST and enter the address of your Web.api controller (“http://yoursite.com/api/yourControllerName”).
3. Again, under the same tab, In the Request headers Text Box Area (The heading 'Request Headers' may not be visible), enter a JSON content type. “Content-Type: application/json; charset=utf-8″ (w/o the braces).
4. Again, under the same tab, Put your JSON string array (Input param) in the Request Body. e.g., ["aaa","bbbbb","cccccccc"].
5. Click on "Execute". it will hit the controller and you can debug.
To Filter the huge number of network communications recorded in the Fiddler
1. Click on the "Filters" tab, Select "Use Filters"
2. In the 1st drop down under the Hosts Heading, Select "Show Only Intranet Hosts", in the second drop down, select "Show only the following Hosts"
3. In the following text box enter: Enter the local url for e.g. "http://localhost:31314/", here, I was hosting the service in IISExpress.