Add CDept_Id In first query for result

In my first query, I want to get CDept_Id. But CDept_Id column does not exist in inward_doc_tracking_hdr table.

It comes from inward_doc_tracking_trl table. like below

select CDept_id from inward_doc_tracking_trl where ref_mkey in
(Select mkey from inward_doc_tracking_hdr where doc_no = 'IW/HU/16/42')

So, From this. I get CDept_Id. Now I want to add this in my below query.

Select mkey,Delivered_By,Department_Id, 
 (Select mkey from erp190516.dbo.emp_mst where mkey  in 
 (select employee_mkey from erp190516.dbo.user_mst where mkey =   To_User))User_Id, 
Doc_Type,Email_Id,Ref_No,convert(varchar(25),Ref_date,103) Ref_date,
Inward_Amt, Remarks, party_name, disp_through
from erp190516.dbo.inward_doc_tracking_hdr, CDept_id  -- add CDept_id here
 where doc_no = 'IW/HU/16/42'  
 and Status_Flag = '13'

How to add this

UPDATE

inward_doc_tracking_hdr mkey is equal to inward_doc_tracking_trl ref_mkey