How can I create an APFS fusion drive in Mojave using my SSD and HD?
Nothing could be easier with Mojave : just enter the following command at the command prompt 

diskutil resetfusion


The process is done completely automatically. Just take care that the detected hard drives solid state and mechanical are correctly determined before answering Yes.

Sample command output : 

-bash-3.2# diskutil resetfusion

Internally-located hardware disk devices known to the currently-running macOS:
Rotational                               (disk0)
Solid State                              (disk1)

Volume exported by partitions or storage systems hosted on the above devices:
Macintosh HD                             (disk23)

WARNING: All of the above will be erased
Do you want to continue? (Enter "Yes" to proceed to erase) Yes

Forcing unmount of all volumes on the solid-state (SSD) disk drive
Started on disk1
Finished on disk1
Forcing unmount of all volumes on the rotational (HDD) disk drive
Started on disk0
Finished on disk0
Partitioning the solid-state (SSD) disk drive
Started on disk1
Unmounting disk
Creating the partition map
Waiting for partitions to activate
Finished on disk1
The partition that will become the solid-state Core Storage PV is disk1s2
Partitioning the rotational (HDD) disk drive
Started on disk0
Unmounting disk
Creating the partition map
Waiting for partitions to activate
Finished on disk0
The partition that will become the rotational Core Storage PV is disk0s2
Creating a Core Storage Logical Volume Group
Started
Unmounting disk1s2
Touching partition type on disk1s2
Adding disk1s2 to Logical Volume Group
Unmounting disk0s2
Touching partition type on disk0s2
Adding disk0s2 to Logical Volume Group
Creating Core Storage Logical Volume Group
Switching disk1s2 to Core Storage
Switching disk0s2 to Core Storage
Waiting for Logical Volume Group to appear
Discovered new Logical Volume Group "515C316B-D0E8-4507-AF51-3B7F6F65B74A"
Finished
The new Core Storage LVG is 515C316B-D0E8-4507-AF51-3B7F6F65B74A
Adding a Logical Volume to the Core Storage Logical Volume Group
Started
Waiting for Logical Volume to appear
Formatting file system for Logical Volume
Initialized /dev/rdisk23 as a 571 GB case-insensitive HFS Plus volume with a 49152k journal
Mounting disk
Finished
The new Core Storage LV is 60BD449A-F95B-49D7-B55D-81978D4567FF (disk23)
Confirmed that your new volume "Macintosh HD" (disk23) is a Fusion Drive
Operation successful