Edit A Drop Down Box In Asp

I need to be able to edit a drop down box is asp. On my web app I
populate a drop down box from a databse. The user can then select an
option that will then be saved to a Database for example

Mr
Mrs
Miss

but if the option is not their e.g Dr, Prof etc then I want them to be
able to enter in the drop down box and then I will save this to the
database.

I know that I will have to have a function to dynamically re-populate
the Drop down box to include this option once the new value has been
entered, so that it will be included in the POST method.

But I can't even edit the text. I have tried

<SELECT>
<OPTION selected contenteditable=true> value="Mr">Mr</OPTION>
</SELECT>

and

<SELECT contenteditable=true>
<OPTION selected> value="Mr">Mr</OPTION>
</SELECT>