I can't set a default value on hook_form_alter()
According to this Documentation you can do something like  

 $form['field_code_postal']['widget'][0]['value']['#default_value'] ='COUCOU1';


and for another guys help if you want to fill entity referenced field( also files are entity in drupal 8) use 

 $form['field_image']['widget'][0]['target_id']['#default_value']= $file; //