Currently the ComboBox widget (applicable for AutoComplete, DropDownList, MultiSelect too) is designed to show the end user a suggestion box with items. It is not designed to allow create/update operations through the UI, this functionality cannot be accomplished out-of-the-box. What you can do to achieve your goal is to provide to the user an additional form, which will give him/her the ability to add new item. Once it is added, you can rebind the combobox and select this new item. Here are some helpful links:
# get widget data source
# add item to the data source:
# set widget value
Please note that this functionality is a custom solution and its implementation is up to the developer
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