This is just fast demo. You can use new ID for insert for update, insert into another table, query, etc. in another way. Hoping I did not insert errors into script during formatting, editing post
-- run [1] before this script once to have environment
--create temporary table once if not dropped after
-- really only ID field is needed, the others are for illustration
create table #temp_id (Id int, d1 int, d2 int)
select * from Table2;-- this is read-only, filled once here source
select * from Table1;--interesting for following runs
insert into Table1
OUTPUT INSERTED.id
-- really only ID is needed, the rest is for illustration
, inserted.d1, inserted.d2 INTO #temp_id
select field1, field2, null-- null to be merged later
-- or inserted/updated into another table
from Table2;
select * from Table1;
select * from #temp_id;
MERGE Table1 AS TARGET
USING #temp_id AS SOURCE
ON (TARGET.id = SOURCE.id)
WHEN MATCHED
--AND OR are redundant if Table1.ID is PK
THEN
UPDATE SET TARGET.IDnew = SOURCE.id;
select * from Table1;
--drop table #temp_id
--drop table table1
--drop table table2
[1]
Reproducing the tables from question and filling with data
create table Table1( Id int identity primary key, d1 int, d2 int, IDnew int)
create table Table2( field1 int, field2 int)
insert into table2 values(111,222)
insert into table2 values(333,444)