As of version 126.96.36.199 we can use Get<Entity> Action as a function in the expression editor, so we can assign a value like this:
The problem is that this Action/Function is not optimized to just fetch that specific attribute - It will return the whole entity, meaning all attributes. This can have a meaningful impact on the performance of your app and as such, is not recommended.
My Idea is to improve Get<Entity> Action/Function to fetch only one attribute. This means that to be used as function, there would have to be a new Entity Action called "Get<Entity>Attribute"