create a select box in which data is populated from selecting another select box and it is editable

Question

I have created two select box. in both, fetching data from database. By selecting first select box the second select box gets auto populated. What I want to do is that this second select box should be editable that is the user can type input also. What I have done now is that used datalist for second select box which is doing the thing, but I want to show the auto populated data as selected option in datalist