Swipe left on JuiceSSH's popup keyboard to use the autocomplete input field.
I've been using JuiceSSH about a year now (as of 2018-09) with tmux & irssi and have only just realised that there's an autocorrect/autocomplete area hidden in the pop-up keyboard. >_<
I now realise the changelog from October 2014 mentions it as well as the extremely lengthy FAQ (it should probably be a second manual...), but this feature isn't made obvious through the JuiceSSH website screenshots nor discoverable design indicators within the app (think SwiftKey's red dots).
The reason I didn't read the FAQ entirely was due to its size and the apparent straightforwardness of everything else - see also: https://www.improbable.com/ig/winners/#ig2018 (Ig Nobel Prize Literature Prize for 2018)