In BikeCAD version 18 we can design a bakfiets style cargo bike. You'll find the option for this in the headset dialog box. Maybe not the most intuitive location for this control, but this style of bike does have two headsets so we'll start there. Once the bakfiets option is chosen, we'll find a new section in the tubing dialog box to configure the middle head tube, the length of the front head tube and the distances in X and Y coordinates separating the bases of these two head tubes. Once you've spaced out the head tubes as desired, you'll likely want to connect them and you can do that by adding an extra tube from the middle head tube to the head tube. You can also go to the rack and cargo section and add a bucket. The bucket is located by X and Y coordinates from the base of the middle head tube. For non-bakfiets designs, this bucket option is still available but it will be located with the respect to the base of the regular head tube and you'll likely want to make it a lot smaller.
When designing a bakfiets, it is worth noting the six methods we have for controlling the front end geometry of the bike. Remember that when you select one of these methods, you are choosing to drive the front end geometry by a single pair of dimensions. The other five pairs of dimensions will behave as driven dimensions. You can select almost any of these options and toggle smoothly between a regular bike design and a bakfiets bike design. For example, if we choose the stack and reach method, when we toggle from regular bike to bakfiets, our bakfiets will have the same stack and reach we started with. The one pair of dimensions you'll likely not want to use when toggling between regular bike and bakfiets is the front center distance and head tube length. Your bakfiets will need a much longer front center distance, so this is not a dimension you'll want to hold constant when toggling between these two styles of bike.