@tuonoman
That image is what I was looking for on first reply.
So, you do have a combobox...that will allow searching.
Next, you are trying to set the Default property on the Combobox...which does nothing. The only Default formula that is of value to the Combobox is DefaultSelectedItems.
You MUST supply records to the DefaultSelectedItems of a ComboBox, In your formula, you are supplying a Text value...this will not work.
Your formula on the DefaultSelectedItems property should be:
Coalesce(LookUp(TAG; TAG_ID=LISTEPRODUCT.Selected.ID_TAG); Parent.Default)