1. Open link to Hipmunk...
2. Copy the link to United and paste it to a new tab
3. Wait until United is showing your routing (You can see the POS in the upper right)
4. Simply add &POS=GB to the current URL (Replace GB with your local ISO2-Country code.. but be aware that not every country is available) and submit
5. You will be redirected to leg-selection (Desired routing likely won't show up)
6. Just close the current United tab/window
7. Copy and open link from hipmunk again
8. You should be fine and POS should be set