why not have do something like the following, since this is really going against the record filter for the report:
local stringvar param:={@parameter};
local numbervar asterick:=instr(param, "*");//returns 0 if no * is found
if asterick = 0 then asterick = len(param) + 1;//adjusts for if no asterick is in the parameter
if asterick > 0 then param = left(param, asterick-1);//what are the letters chosen?
left({table.field}, asterick-1) = param //return true(include in report) if the field starts with the same letter, and false(exclude) if it doesn't

I am not sure what would happen if the user just enters *, but for all other occasions this should work, whether the user uses the asterick or not (wants an exact match)

HTH