If the task list doesn't exist yet, then use IBIP (or IBIP via LSMW).
If the task list already exists, you may update the text with SAVE_TEXT (see other posts in the forum which explain what is needed around) + you need to have the long text indicator set, as explained in note 1982719 - Long texts cannot be displayed - Usage of function module SAVE_TEXT.
When i am trying to update it from SAVE_TEXT FM then its updating and i can read those texts from READ_TEXT FM also but these are not displaying in IA02. I am trying to create the operations long text and short text is already created. Any idea how i can do this task or is there any other FM or BAPI ?
You may update the text with