You can set up an 'other' option in dropdown lists to allow users to type something manually if there is nothing else in the list that suits.
i.e. You could set up a dropdown list called 'choose' your favourite colour with the following four options:-
If the user selects 'Other' a text box would appear with a label "Enter your other choice here" allowing them to type an answer in.
To set this up, create a dropdown list as normal.
For the final entry called 'Other', simply add the text "Enter your other choice here" into the reason box.
NB - it does not need to be the last item in the list that has the reason text. You can also have multiple entries in the dropdown that offer this same facility.