Create a drop down input box when creating a task

Hello Experts,

I would like to do the following with your generous help:

1) Select a button which creates a task from an existing e-mail
2) Upon creation of task, have an input box that is a drop down list which is stored as part of a text string that ultimately becomes part of the subject line of the task.

WHERE I NEED HELP, MY DEAR EXPERTS:
a) How to create this input box drop down
b) Where/How to maintain the list of values that appears in the drop down.  This would be a manually managed list which I am assuming I can just edit within a given VBA module.

Thanks so much.  I look forward to your replies.