How to fill custom fields with brackets in their key with add_post_meta()?

This is the PHP serialization format, and is used to serialize arrays for storage in the database. Core WordPress functions take care of this process for you. You just need to use an array as the value.

For example, this:

$schema_article = array(
    'headline' => 'aaa',
);

update_post_meta( $remote_id, '_schema_article', $schema_article );

Will save this value:

a:1:{s:8:"headline";s:3:"aaa";}

If you want to see the original array for a value, get_post_meta() will unserialize it for you.

For example, this:

$schema_article = get_post_meta( $remote_id, '_schema_article', true );

Will return (as seen via var_dump()):

array(1) {
  ["headline"]=>
  string(3) "aaa"
}

So when saving the value you need to mimic the original array format. I would share what that is for your example, but it cannot be unserialized because it has been corrupted by editing the value by hand.