Search duplicate record on data entry (1 Viewer)

I realise this is an oft covered topic, but I've had trouble implementing the answers of found on other threads.

I'm wanting to check if the entered "CHI", the primary key (text, not number) is already contained in the table immediately after update (or before?) of that field, i.e. just after the CHI has been inputted, and well before the rest of the data is input into the form and the 'save' button reveals the impending error. This is simply to save time for users.

Thanks guys

If Dount("CHI", "Table1", "[CHI] = Forms!Form1.txtCHI") > 0 then
Msgbox "Error - that CHI value is already in Table1"
End if

The above is a very simple way to check - you'd need to replace Table1, CHI, Form1 and txtCHI with the relevant values though.

You can have an unbound box,the only box on the form.
Enter CHI,
upon enter, it will either open the detail form full of existing data,
Or
open it empty ,ready for data entry.

thanks dairees, works perfectly. Thanks also Ranman256 for your response