Is it really worth it to save one line? I guess if you really want to be hacky, you could do this:
var myKey = 'text';
var myObj = JSON.parse( '{"' + myKey + '": "Hello"}' );
alert(myObj.text);
I would actually just declare the object and set the key
var myKey = 'text';
var myObj = {};
myObj[myKey] = "Hello";
alert(myObj.text);