I Want to make some dropdownlist act like a textbox without losing the ability to explore the list and i want the list to autoselect the list item that match the text in the textbox (Searchable DropDownList).
if the text didn't match any list item i want to insert this value into a lookup table.
I'm using the AutoCompeleteExtender in other textboxes but with this extender the user unable to open the list and select a specific item.
if there is a better practice i'll be glad to know it.
I personally recommend the Select2 plugin, I have been using this in most of my project which have the same requirement as you have.
Select2 gives you a customizable select box with support for searching, tagging, remote data sets, infinite scrolling, and many other highly used options.