How to Add T-Code to Every SAP Users Favourite?

Use GUI_ADD_TCODE_TO_FAVORITES function.

SELECT *
    FROM usr21.

    CALL FUNCTION ‘GUI_ADD_TCODE_TO_FAVORITES’
      EXPORTING
        transaction_to_add = p_tcode
        user_name          = usr21-bname
        no_par_tcode       = ‘X’.

ENDSELECT.