- Go to the Humble Bundle website
- Make sure you are logged in
- Press
F12
to open the Developer Tools - Go to the Application tab
- On the left pane, Select Cookies and then https://www.humblebundle.com
- On the right pane, click on the row that has _simpleauth_sess as its first column
- You can copy the cookie value from the pane down in the Developer Tools
For a visual guide on steps 4 to 7, see this picture.