我在哪里可以找到页面的phpsessid（它既不在cookie中也不在URL中）？

Question

I am currently trying to automate some actions on a website only using http requests. I'm using nodejs for the bot and so far I had no problems with cookies, csrftoken etc. But for one request, the browser sends a phpsessid within the cookies

cookieStringA   7163240,7166759
dev_ref_no  vfmTVB
login_token cfd3deexxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx3eec62
PHPSESSID   e2xxxxxxxxxxxxotv1
sec_advice  sec_advice
X-Access-Token  e84fxxxxxxxxxxxxxxxxxxa001
X-Client-Id wg_desktop_website
X-Dev-Ref-No    vfmTVB
X-Refresh-Token 495xxxxxxxxxxxxxxxxxxxxxaa85

copied from firefox.
I googled around and read everywhere, that the phpsessid is normally stored in the cookies, but it's neither the request cookie jar (all other cookies are there) nor can I find it in the browser when using document.cookie.
But on the same page, when I click the send button, the request has this phpsessid.

The website I'm using this on is www.wg-gesucht.de
Do you guys have any hint on where I can find this phpsessid?

Thanks in advance, T0Bi