Enzo,
How are you selecting the microflow, are you populating the reference selector with a microflow? Then you might have an issue in the way you're getting the data.
Are there restrictions on the reference selector? Could be that these are setup incorrectly.
Are there on change actions on your reference selector, that influence the content of the field?
These would be things I would check as this association is the same as any other association. I have created an example of the thing you'd want in a couple of minutes without the issue. If you want I can share this, it might help in finding a solution to your issue.
You can not associate a microflow to an entity in Mendix.
You can create an event handler in the entity, but only on create, delete, commit and rollback.
I am not sure what you are trying to accomplish. Did you do the mendix introduction course yet? https://developers.mendix.com/training/