Setting SameSite=None on a session cookie

I have a custom endpoint which stores a value in a session cookie, using this code:

$session = $this->request->getCurrentRequest()->getSession();
$session->set('test.email', $data['email']);

I'm calling this endpoint from a decoupled frontend.

This works fine in Firefox, but in Chrome it looks like the session cookie is being blocked because SameSite is set to Lax. How would I set SameSite to be None? Ideally I only want to do that for this cookie, not all cookies.

You can change the session cookie settings in services.yml. See How do I set the cookie lifetime?

This is not only possible for the default parameters defined by Drupal but for any parameter listed in the PHP docs:

https://www.php.net/session.configuration

So you can add SameSite=None to the session storage options:

sites/default/services.yml

parameters:
  session.storage.options:
    gc_probability: 1
    gc_divisor: 100
    gc_maxlifetime: 200000
    cookie_lifetime: 2000000
    sid_length: 48
    sid_bits_per_character: 6
    cookie_samesite: none

These options only apply to the session cookie, not to any cookie set by your site. To change the options dynamically see How do I dynamically change cookie_lifetime?