Yes, it is possible to enter remark for a complimentary item.
Refer to the below screen-shot to configure the setting.
Please follow the below steps for the same:
eZee POS Back Office>>Configuration>>Outlet Operations. Select the outlet>>Micsellaneous.
Kindly, checkmark the option " Ask Reason On Complimentary ".
And to enter remark for a complimentary item in eZee Burrp we need to click on options than on complimentary.