Hi Noura,
If you use the standard Save button, you should use the default Reference (set) selectors or Marketplace widgets over the association. If you use a dropdown widget that selects values but is not over the association, you should change the Save button into a MicroFlow where you can process the values from the dropdown according to your need. In that case, right click the button and select “Edit on click action ..” to “call a microflow” and create a new microflow.
Go Make It