Hi Ma zong,
I might have misunderstood your question, but here is what you could do;
I created an entity for the testing purpose
And a microflow that just creates an instance of above entity. This is then passed to a page to have a starting point.
Here you see that I used the instance I just created in the microflow before to show a dropdown to the user. There also is a button which I named 'Select' If this button is clicked, the following microflow is called:
Long story short: You can use an exclusive split to handle the values in enumeration attributes.
The dropdown selection is passed to the microflow.
Hope this helps