Alternative way to access a field in a %ROWTYPE variable

Hi,
Is there another way to access to the field in a variable of type record?

I need to store the name of a field in a variable and access
the field whose name matches the content of the variable.

Just as an example of what I want to do, the table or view "pg_user "
has at least the field usename, usesysid, valuntil

So I need to access the fields in a way similar to this
DECLARE
row pg_user%ROWTYPE;
field VARCHAR';

BEGIN
field = 'usename';
row.field = 'name';
field = 'usesysid';
row.field = 0;
field = 'valuntil':
row.field = 'who knows';

Is it possible to do something like that?
because that example doesn't work

I'm using plpgsql;

tanks in advance
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