This increase is worked pretty much like Tunisian full stitch (Tfs) in that the hook is inserted between two stitches under the entire horizontal chain through the whole fabric:
When used as an increase, it is not worked in any specific stitch of the previous row, so the two stitches are the one just worked and the following stitch.
Insert your hook under entire horizontal chain in the space before the next stitch. Yarn over and pull up a loop:
Leave the loop on the hook: