Hey teuku ryan!
A way to achieve what you want is this one. As you are using non persistent object I assume that the reference selector is using a microflow as Datasource.
To set a default value, before entering the page you need to fill the association with the object you want, so before going to that page you should do a change and set the attribute association you are using on the selector:
With that you should have a value selected when you enter on the page. However, you need to take into account that this object that you have on the association should be on the list returned on the selector datasource microflow(first image) . So on the microflow you should retrieve the current value and add it to the list cause if you don't do that you'll see an empty value on the page.
Hope this helps you. If you have any question about this, please let me know.
nvm i already found solution