how to add foreign key to table using postgres code example

Example 1: create table postgresql foreign key

CREATE TABLE so_items (
	so_id INTEGER,
  	...
	FOREIGN KEY (so_id) REFERENCES so_headers (id)
);

Example 2: psql create table foreign keys

# id_user is the primary key of the table users:
create table lists(
id_list serial not null primary key,
id_user int references users(id_user),
is_temp int
);