I’ve been noticing some chords in recent TheoryTabs that function similar to how V/V & IV/V chords work, but with ii instead. Is this a new feature? What buttons on the Hookpad interface allows me to use them?
https://www.hooktheory.com/theorytab/view/electric-light-orchestra/mr-blue-sky
Search/type the chord label directly (for instance, ii/V) or copy it from another theorytab which uses it (I hope that they will also add subV and N6, Gr+6, Fr+6, It+6 would be great as well)
Newbie question: What do you mean with “Search/type the chord label directly (for instance, ii/V)”. How can I type in a chord label directly?
And: I need that subV too!!! 
Sorry, found it myself – after buying Hookpad for at least a month 
But even with search/type I can’t get every chord. Especially substitute dominants are missing, f. e. Cmaj7 – Gb7#11 – Fmaj7 = I – subV/IV – IV.
(I would write that sequence: I – bII7 –> IV, but that is another story … ) 
I made a request for the subV long time ago but I think that never would they add it (not really sure about the reasons but I think they once said that they want hooktheory/hookpad to remain a generally pop/simple based platform due to the fact that the majority of the users are not classicaly trained or jazz-oriented musician)