Immutable struct Property Drawer

I have a custom struct that is immutable: all its properties are private, and it is assigned as a numerical value. How can I make a PropertyDrawer assign a new values directly to the property it's drawing?

I heard I should have the drawer started with something like this :

 public override void OnGUI(Rect position, SerializedProperty property, GUIContent label){
     object target = property.serializedObject.targetObject;
     MyStruct? value = target as MyStruct?;
 } 

...but value always ends up being null here.

Then after I manage to grab the value of the target object, how can I send it back into the target object ?