Hi,
This is a very specific use case.
The coupon solution can be a good idea, but actually we can't set the coupon required.
It will require custom code to edit that.
The password solution in a custom field seems to be a good idea too, but it will require too custom code.
I think that a plugin can do the job, base this plugin on the trigger "onBeforeOrderCreate()".
If the password is correct, so let do hikashop create the order, else set the var $do to false to not create the order.
Here is some documentation about plugins in HikaShop:
www.hikashop.com/support/documentation/6...#onBeforeOrderCreate
It require some PHP knowledges.