Add a column with Not Null

I have a existing table in which i am adding a column as NgoId whose datatype is int. I want to add as NOT NULL to the column .

When I'm adding this I am getting the following error:

Unable to modify table. Cannot insert the value NULL into column 'NgoId', table 'CSR.dbo.Tmp_tbl_post_category'; column does not allow nulls. INSERT fails. The statement has been terminated.

Please help how to add the column with NOT NULL.