Just for jollies, I created a composite glyph in the private use area @ $FB07 with a name of “abform”. This glyph was a composite made of the characters “c” and “d”. In the OpenType Designer, I added a Standard Ligature 1 feature (liga) and then I added a lookup table Ligature 1 and then I added the lookup into the table. In adding elements to the lookup table, I was not able to enter “ab” in the left-hand column so that I could point it to the “cd” ligature. However, after setting up the a → cd lookup, the script editor enabled me to add the b resulting in “ab → cd”.
You can see in the screen shot that where ever “ab” is encountered in the input stream, those two characters are replaced with “cd”.
Good Luck
The following may make this all a little clearer: Tutorial