Seems it's a limitation
please see the below community thread
21R1: Cannot use custom attribute as child entity key in key mapping for entity association | IFS Community

Hello Kelum, thanks for the answer. Though this thread is not accessible to me but I think the work I am trying to do is currently not possible. Can you guide me any other way to associate two different entities. Thanks again for the guidance.

I made a quick test and it seems like it is ActiveSeparate that does not work. It is because the base view name and table name is not the same for that LU. View is called ACTIVE_SEPARATE but table is ACTIVE_WORK_ORDER_TAB. I would call that a bug.

Tested with ActiveWorkOrder and that seems to work fine. But using custom fields seems not supported yet.

I don’t know of any workaround since I’m not sure what business need you try to fulfill.

What are you going to use this projection configuration for?

This seems to be still a limitation after 23R1. I cannot see the referred document/link due to access restriction. According to the extract it is stating is the CUD operation cannot be supported and I can understand the complexities.  However, in my scenario I need only to display a standard Entity content and no CUD operations are required. 

Do we know if IFS R&D is planning to expand this in the future?

@anjana.pat 

This is a functionality that we are yet to implement in projection configurations. To have more clarity in this area, we have included this fact in technical documentation under: Tailoring Guide > Configuration > Configuration References > Known Limitations.

The feature is already in our road map, but as of now, we have not planned this for any recent upcoming releases due to other priorities.

However, if your requirement here is only to view data (but no CUD), you can achieve it by creating a Query using Query Designer using the standard entity by including the required custom attributes (Technical documentation: Tailoring Guide > Configuration > Query Designer) & including the Query in your projection using a projection configuration.

Hope this helps!