Solution:

1) In RTL code, assign the following attributes to the unused input:

   /* synthesis syn_force_pads=1 syn_noprune = 1 */

2) For Synplify Pro flow, under Strategy1 make the following setting:

    MAP Design--> Command line options = -u
    This ensures that the Mapper does not remove the unused I/Os.

3) In Spreadsheet View, you can modify the setting as needed.