I presume what you mean is that they will write a letter to say how long you worked there and thats about it.
Yes, you should write a letter to say what you did and describe your job function as much as you can. You should see if someone else of authority in your company can sign such a letter. If not, you could get a job description for a job with the same title from a job ad that your company might have placed. I have no experience in this, i am suggesting stuff that might show your duties. Whether it will work or not is another question. Lets wait for someone else to reply. Perhaps theres someone out there who has been through a similar thing or knows what to do in such a situation.