Easy. Record or enter your melody into hookpad. Figure out in which key your melody is. Choose that key with the “Key” Button. Click “Guides” in the menu. Choose for your melody the chords where those chordnotes already appear in the notearea in shaded colours. You wan´t to match those positionwise. Those chords should already be suggested once you have choosen the right key. Do not hesitate to try fancy 7, and 9… extended chords.
It´s called harmonizing a melody not “Reverse engineering” by the way. You may find a lot of progressions which will work with your melody. Changing a progression over a melody is called reharmonizing. 