How to add a "fillable" field on the timesheet with no specific dropdown values

I am trying to add a blank field to the time sheet that would accept a 4 digit (alpha-numeric) value.  Since the field is 4 digits, the values can vary and the end user knows the value to populate.  I am trying to avoid having a huge list of available values (such as activity) as the maintenance would be too much to maintain.    I also do not need the value to flow to payroll.  It will be used for tracking within time and labor and can be reported on via query (TL_RPTD_TIME).   I cannot find a way to add a