This is one of the ways i use the withy rig, as a combi. Made with stiffish coated braid, a swept shank hook, a rig-ring and some rigid shrink tube.
The popup is attached to the rig-ring via a long hair and is used to limit the length of the hair, the ring slides along the shank of the hook. The whole lot is pinned down with a good sized weight.
The weight is important as it needs to grip the line enough to put some tension on the braid passing through the shrink tube. Just before you cast give it a good tug against the tube, this helps to keep a nice tight curve and stops it getting stretched on the cast (if you look closely you can see how hard i pull it up onto the tube) . Here i'm using one of the fox popup weights which are perfect for the job and are rig safe too.
Once you pull the popup weight tight there should be a few mm of braid showing the other side for the combi to work.
*A note on the long hair; tie the rig-ring on with an overhand knot and use a rig puller on the hair to slide the knot and ring back down the shank to give you enough room when you want to attach a popup, then just pull the rig-ring back up tight to the popup with the rig puller to finnish.
*A note on the shrink tube; I use an 18mm PVA bag filler tube to help form the curve when steaming.
This can be a devastating rig especially it seems with bright popups, this could be because a bright popup often pulls fish down hiding the rig below.
A great alternative which often produces a bite when nothing else will because if this ends up in a carps mouth they won't get it out.