First step:

Ensure current page was [http://www.pixiv.net], Then press hot key [Ctrl+Shift+C], Following Window will show up:

Chrome: Firefox:
Second step:

Click [Network] tab inside Window:

Chrome: Firefox:
Thrid step:

Reload page:

Chrome: Firefox:
Fourth step:

Select [first line] inside Window, then click [Cookies] sub tab:

Chrome: Firefox:
Fifth step:

Copy value of key [PHPSESSID]:

Chrome: Firefox:
Final step:

Paste value to [Prompt], by following format:

Chrome: Firefox: