By configuring this rule in the template when any user raises a ticket with this template they will be set as the requester by default for the ticket.(Applicable for admin, Requesters and technicians)
1. Go to the template you want to restrict the requester>>Click Field and form rules>>On form load>>Create a rule as per the below snapshot.
Script:
var x=$CS.getLoggedInUserName();
$CS.setValue("REQUESTER",x);
2. To disable the requester field selection -->Create another rule in on form load>>Create a rule as per the below snapshot.