how can I reference another column inside postgres insert query?

I have the following table:

CREATE TABLE tab (
    id SERIAL PRIMARY KEY,
    code TEXT NOT NULL,
    data TEXT
)

In some cases, I'd like to insert a new row ensuring that the code column is generated by the id column. In other cases the code is provided by the user.

For example:

INSERT INTO tab(code, data) VALUES ('code: ' || id::TEXT, 'my data');

The expected result is something like:

INSERT INTO tab(code, data) VALUES ('user.provided.code', 'my data');

The expected result is something like:

Is it possibile in one statement?

It sounds like you want to default the coder to something based on the id. Unfortunately, this doesn't work in Postgres:

create table tab (
    id   integer primary key generated always as identity,
    code text not null default ('code '||id::text),
    data text
);

One option is a single statement that does both an insert and update:

with i as (
      insert into tab (code, data)
           values ('', 'my data'
           returning *
     )
update tab
    set code = 'code: ' || id::TEXT
    where tab.id in (select i.id from i);

Another is to use a trigger that assigns the value.