It sounds strange that you cannot find the information in the resource shape. I suppose you get confused with what the "prefix" is - it is simply a namespace declaration, or a "shorthand" if you like. Here is something you can do, and hopefully get what you want.
1. Follow the usual chain of discovery till you get the resource shape of the defect. In the shape content, search the custom attribute name and you should get something like this

<oslc:propertyDefinition rdf:resource="http://jazz.net/xmlns/prod/jazz/rtc/ext/1.0/customAttribute"/>

Here the name space is http://jazz.net/xmlns/prod/jazz/rtc/ext/1.0/ and you need to declare this before you can use it in a request body.
2. Depending on the name of your choice, you can declare the namespace as below

xmlns:rtc_ext="http://jazz.net/xmlns/prod/jazz/rtc/ext/1.0/"

In this case, you can use <rtc_ext:customAttribute> to refer to your custom attribute in your request body.
3. If you want to get a sample of the RDF/XML presentation of a Defect work item, simply do a GET request of an existing Defect work item with the custom attribute. At the top part of the response, you should observe the same line as quoted above. All the attributes with the "rtc_ext" prefix are custom attributes. The other attributes have different prefix depending on in which namespace they are defined.
I don't know how you do the "fetch", but to get consistent result, always use HTTP headers "Accept: application/rdf+xml" and "OSLC-Core-Version: 2.0" with your request.