There's no special F.8879 for 1040X at this time.  It will be the same F.8879 as if the 1040X were an original return.  The numbers should jive and you can make a notation that the F.8879 is for 1040X.

If you do not see the F.8879, it is most likely because you didn't check the SELECT FOR E-FILE box for Federal Amended under Profile.  So long as that box is checked, the authorization form should be produced automatically and you can print it like from your usual returns.

Note, however, that Intuit doesn't support e-filing of 1040X if the original return was not previously e-filed with the program.