Syntax question about returning value from an insert

I am writing a trigger/function to make certain a default item, and its key
exist when an insert is called. EG

The trigger gets called on insert to T1 If column c1 is NULL in the NEW
structure, I need to check table t2 to get the key associated with the
default for this column. However, if the default is not yet inserted into
t2, I an to go ahead and insert it.

I found this page:
https://stackoverflow.com/questions/6560447/can-i-use-return-value-of-insert-returning-in-another-insert
which sugest this syntax:

with rows as (
INSERT INTO Table1 (name) VALUES ('a_title') RETURNING id
)
INSERT INTO Table2 (val)
SELECT id
FROM rows

I modified it slightly to look like this:

IF _bom_name_key is NULL
THEN
with rows as (
INSERT INTO project_bom (bom_name) VALUES ('Main') RETURNING
project_bom_key
)
NEW.project_bom_key = SELECT project_bom_key
FROM rows ;

But this gives me syntax error.

I realize this functionality is slightly different, but can I get the new
key into the NEW structure to return from the function call?

--
"They that would give up essential liberty for temporary safety deserve
neither liberty nor safety."
-- Benjamin Franklin