Define the object before the $.post() call. A little something like this:
// variable somePropertyName holds the name of the property you want to set
var myPostParams = {};
myPostParams[somePropertyName] = "some value";
$.post('page.php', myPostParams, function () { /* etc. */ });
Note that if some of the parameters you are passing will be constant you can define them up front:
var myPostParams = {"x" : "something", "y" : "something else"};
myPostParams[someVariablePropertyName] = "another value";
myPostParams[someOtherPropertyName] = aValueInAVariable;
//etc.