Define a Keyboard-String Input response, call it, say, “answer”; leave the “The response” blank; and check the “Save this response to the collected data file” option. Select “Show String Input’s Edit Field etc.” = “Only if an event designates etc.”. In Reponse Completion, specify no limit on length of string, a liberal time-out period such as 30 s, and select “If the participant presses the Return/Enter key” = “Participant has completed the response”.
Define a preliminary event “study” to display the list of study items as a text string for a certain time.
Define a test event “test” to display the cue (e.g., Ar…) as a text string; click the “Record and save response” button; set up Input to “End this Event…After any response”; and set the Correct Response to “One or more of the following”, and click the checkbox next to “answer”.
Set up a trial “the trial” to include “study” and “test”, and set up a block “the block” to include “the trial”.
Then click 
to run everything. You’ll see the study set for the specified time, then a screen with the cue (Ar…) with a string entry box under it. The subject types “Armed” (which echos on the screen) and hits return, which ends the event, the trial, and the block. If you saved the data in a file, you’ll see Armed as the subject’s reponse.
Does this not meet the criteria? Or have I misunderstood?
The difference, as I understand it, is that in the originial specification, the subject would only type “med”, and the program would combine that with “Ar” to form “Armed”. But in my previous suggestion, I proposed that the subject type the whole word, that is, “Armed”; that way, no later combination would be required; the subject would simply see the whole word as he or she typed it. If this isn’t adequate, then of course my suggestion doesn’t work.
However, there is another possible approach if the subject must type only the completion, and that would be to position the cue string immediately to the left of the keyboard string input box, so that the letters typed by the subject would appear immediately to the right, forming a word. That would be tedious to set up, but it might be quite satisfactory.
Greg Shenaut