Disable Scan Shift Enable in Functional Mode

Hello,

I'm trying to disable shift enable in function mode.  

This is the VHDL code:
 i_scan_shift_enable <= '0' when scan_mode_in = '0' else scan_shift_enable;
   

So "scan_shift_enable" is driven from a pad and  "i_scan_shift_enable" drives the flops.

However I can't seem to keep net i_scan_shift_enable preserved and even if I did would it be a valid pin for the DFT tools to hookup to.

How is this generally done? Is it necessary to instantiate tech cell, preserve and hookup to the output pin?