Replace a ID field with a combo box field

[john john]

I have 2 SharePoint lists; Assets & Media.. Inside the Assets list i am storing the MediumID inside a number field.. now inside the Assets form i want to replace the MediumID with the combobox that contain the Media items to select from.. for both NEW Form, Displayform and EditForm..

so inside the Assets form i removed the MediumID number field >> and i added a combobox named "MediumCoboBox", with these values:-

and inside the MediumID data card  added the following Update property:-

this is working for adding new Assets,,, but when editing or viewing the Assets inside the AssetForm the MediumCombobox will always be empty.. so what i need to define inside the Default() property?

Thanks

[#2]

ThisItem.xxx

[#3]

@SaniGarba what do you mean exactly ? Thanks