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get thet typed or the selected item

I don't understand how the server get the selected or typed item of the combobox.

I use a form with a select box

If combobox placed inside form tag in will send data as default select box control  ( as part of form submit )
If you creating combobox from selectbox it will take the same name as source selectbox.
In other case you can set used name ( which will be available for server side code as second parameter of constructor )
    var combo = new dhtmlXCombo("myDiv","myName");