The correct format for the Rest API POST Search Query is:

 {
 'request': {
 '__metadata' : {
 'type' : 'Microsoft.Office.Server.Search.REST.SearchRequest'
 },
	 'Querytext': 'query'
 }
 }


I found an example [here][1] in the second image.


 [1]: http://sp2013searchtool.codeplex.com/