Hi Jampani Sridhar,
You have to add one field in your HCM F&P "FORM_SCENARIO_STAGE TYPE ASR_FORM_SCENARIO_STAGE and Give Field attribute as "Autom.(Step Relavant).
Now in your Adobe Forms, write following code:
if($record.CONTROL_PARAM.ISR_MODE == "APPROVE_REQUEST")
this.access = "readOnly";
endif.
This will not allow initiator to Enter Any Data in Drop Down.
Thanks & Regards,
Tushar Trivedi.