This worked for me inside a javascript button :

var obj = this._contextObj;
var guidEntry = obj.getGuid();
mx.data.action({
    params: {
        applyto: "selection",
        actionname: "MyFirstModule.MyMicroflow",
        guids: [guidEntry]
    },
    origin: this.mxform,
    callback: function(obj) {
    },
    error: function(error) {
        //alert(error.message);
    }
});

I remember that I had a problem using “var obj = this._contextObj;” inside a html snippet (it wasn’t working), this is why I had to use the javascript button