<TelerikComboBoxData="@_List" @bind-Value="aItem"Placeholder="press return to add a new item"AllowCustom="true"OnChange="@OnChangeHandler"></TelerikComboBox>
the user can write in the combobox, hit return and then the new item is in the list
if you want to edit the item - I would use a textbox outside of the list, or after editing in "OnChangeHandler" ask the user, if the item is new or changed