QuickBuild doesn't support json rest api, now.
I found the documentation of RESTful API regarding 'Interact with Build Requests' like shown below.
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Since QuickBuild 6.0.14, one can also trigger new build via GET request by accessing below url:
http://localhost:8810/rest/trigger?configuration_id=
Where should be replaced with identifier of a configuration. The response is of mime type text/plain and the content is XML representation of newly created request result. Note that this approach does not accept variables or promotions information.
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Why don't you add json rest api feature in QB?
If QB supports it, people can make an build request more easily with some variables for specified configuration without creating xml file.

For example,
$ curl -X POST http://localhost:8810/rest/trigger -H "Content-Type: application/json" -d '{"configuration": "1234", "var1": "var1_value"}'