Ok, well we can do that too 
So if we allow searching, but we don't want the item to submit unless there is a selected value, we set the field to required, or we can change the Submit action.
For your Submit button, have it check:
If( IsBlank(Combobox1.selectedItems.Property),
//If blank then let them know they need to choose
Notify("Please choose an item from the dropdown", notificationtype.warning),
//If not blank, ie an item is chosen, then submit the form
Submit(FormName)
)
NB - Property will need to be the value or thing within that selectedItem like .name .Title .displayName etc - use intellisense to figure out which will work for you.
you can check this with a label during testing:
Could you give something like that atry and let me know if you run into any issues or if it solved your problem?
Cheers,
ManCat
