Hi Parvez,
Please ensure that you want to write in EH_ONSAVE only as it will get triggered only at the time of save button press. If you want it to be happening always while the save, go to order_save badi as Luis suggested. There you can call the FM CRM_ORDER_READ to get all partners in et_partners.
If you are sure to write in eh_onsave (as your requirement is only for save button press), you can try something like below.
DATA: lr_btadminh TYPE REF TO cl_crm_bol_entity,
coll TYPE REF TO if_bol_entity_col,
entity TYPE REF TO cl_crm_bol_entity,
lr_partner TYPE REF TO cl_crm_bol_entity,
lr_partner_all TYPE REF TO cl_crm_bol_entity.
lr_btadminh ?= me->typed_context->btadminh->collection_wrapper->get_current( ).
CHECK lr_btadminh IS BOUND.
entity ?= lr_btadminh.
lr_partner ?= entity->get_related_entity( iv_relation_name = 'BTHeaderPartnerSet' ).
lr_partner_all ?= entity->get_related_entity( iv_relation_name = 'BTPartnerAll').
Now lr_partner_all will have all partners. You can loop it and read by get_property_As_value.
Thanks,