there are a few methods of finding the suffix for a specific voicebank.
the first one is to, in utau, go to Tools > Voice Bank Settings, and then a dialog will show up with all the lines in the oto. you can select one of these lines, and in the box labeled "Alias", it shows the phoneme (eg "u あ") and then the suffix (eg "甘", so the box would say "u あ甘"). you can copy and paste the suffix from there and add it to the end of notes either by hand or through use of a plugin.
you can also view the oto by going to the voicebank files and finding the "oto.ini" in the same folder with all the wav files. it will be pretty much the same thing, just in a different format. the same "Alias" part will come after the "=" and before the first comma (eg "かかきかくかんか.wav=a き甘,[numbers]", and here it would be "a き甘". the suffix would still be "甘").
in some cases, the creator of the voicebank will have configured a prefix.map for it (though it's usually for multipitch rather than multicolor or appends), which you can view by going to Tools > Edit prefix map. you can copy and paste the suffix from there as well.
sorry if this isn't a super clear explanation, my brain isn't working all too well at the moment. if you have any questions i can try to answer.
