it does not allow to add a model attribute because that is already a value node. so add your new attribute and getter and setter methods will be generated automatically.
however you need to know from where the data is coming. when i observed this component they are fetching the data in the method DO_INT_CONTEXT of that view implementation class. look at that you will get an idea.
what i would suggest is redefine this method aafter enhancing the component and view. call the super class method first. then read all records from collection one by one and for each record read the ORDER_NUMBER and using this order number , fetch the requried fied value 'header reference' USING ANY FM ( say order read), Then set this value for the current entity. logic will look like below
in the redefinition DO INT CONTEXT method.
call super class method first.
data: lr_entity type ref to cl_crm_bol_entity.
lr_entity ?= me->typed_context->releasehistory->collection_wrapper->get_current( ).
while lr-entity is bound.
"use get_property method of lr_entity and read order number.
"use order number and get required field value may be using some FM.
"use set the property method of lr_entity and set the attribute (that you added ) with the value (that you fetched above ).
lr_entity ?= me->typed_context->releasehistory->collection_wrapper->get_next( )
end while.
i hope it will work.