For what it's worth, on the latest issue of Unity with the Valiant 25th anniversary variant cover, it notes that Livewire was established in 1993. But as with some other characters, there are significant differences in the old and new characters. So you're looking at potentially two keys. Three, perhaps, if you consider her appearance in #15 of the original series as a cameo, which it pretty much is. She's in action in the next issue.
I may have to pick up Harbinger (V1) #16! Thanks!
If you're looking to collect first appearances, you're probably best off picking up 15 & 16. I've never seen anyone else mention that her appearance in 15 is a cameo, but that's my take based on recently re-reading the issue. (#15 is Stronghold's first full appearance, anyway – he has a final page cameo in 14 – and he does play a role in the current books, but not a huge one yet, so 15 is still key.) Either way, 16 is her first cover appearance.