Mandatory and optioonal parameters mapping issue in Gateway

Hi,

I am facing issue with mapping fields to my RFC(BAPI) in service implementation - FOR Get_entity-READ 

Lets suppose we have 3 Fields as input for BAPI. 2 of which are mandatory and remaining 1 is optional.

Now, Data model->entity type, allows only Primary keys to be part of input paramters to the BAPI under mapping.

Can anyone share how can we pass this optional parameter (in case of Read-Get_entity) to bapi without setting it as Key under entity type.

Thanks in advance,

PavanG