Using a CREATE TABLE AS SELECT how do I specify a WITH condition (CTE)?
It may be awkward, but you have to move the WITH clause from the top into the query. It's a part of the statement to generate the table, and that statement comes after the CREATE TABLE, so you would use this syntax.

CREATE TABLE foo AS
WITH w AS (
  SELECT *
  FROM ( VALUES (1) ) AS t(x)
)
SELECT * FROM w;


Also worth noting that it's not explicit in the official docs, it just falls under query

CREATE [ [ GLOBAL | LOCAL ] { TEMPORARY | TEMP } | UNLOGGED ] TABLE [ IF NOT EXISTS ] table_name
    [ (column_name [, ...] ) ]
    [ WITH ( storage_parameter [= value] [, ... ] ) | WITH OIDS | WITHOUT OIDS ]
    [ ON COMMIT { PRESERVE ROWS | DELETE ROWS | DROP } ]
    [ TABLESPACE tablespace_name ]
    AS query
    [ WITH [ NO ] DATA ]


The docs mention two other use cases for the WITH keyword,


To set storage_options
To set NO DATA


You can see both of them in use like this,

CREATE TABLE foo
WITH (fillfactor = 100)
AS
  WITH w AS (
    SELECT *
    FROM ( VALUES (1) ) AS t(x)
  )
  SELECT * FROM w
WITH NO DATA;


Now you've got a table with nothing.

TABLE foo;
 x 
---
(0 rows)