@Future_Vision
When you add a field to a form, one of the options you have is to select the control type. If you want a dropdown for a text column, you can choose the Allowed Values control type. It will set your datacard up with a functional dropdown.
Note: You cannot change this control type once you unlock your datacard.
But, as you state, search is a feature you want, so a combobox is it.
Now, looking at your properties, the reason you have nothing showing from the record is that you have nothing in the DefaultSelectedItems property.
You are trying to set the default value using the Default property. The Default property of a combobox does nothing. Not even sure why it is there.
You must set the DefaultSelectedItems (DSI) property on a Combobox. And, you must set it to a record, not a text value. The record you set it to must match the record schema of the Items property. (this is why you should only use a combobox if absolutely necessary).
So, that said, your Items property appears to be a sorted list of a data entity called Brands. So, your DSI to be a record that corresponds to that schema.
Tell me more about the Brands list. And also what you want displayed in the combobox (assuming Brand), plus if any other controls or values in your app rely on any values from the combobox (beside the Update property - which is fine because your underlying column is a text column).