I'd like you to do me a favour right now
My guess was that it's a contraction of 言ってくれ, the imperative of 言ってくる.
The person asking the favour is sat next to the other person so くる as 'come' makes no sense. That leaves me with 'start to say anything', which also makes no sense.
I also thought っとく could be a contraction of っておく (I've seen that before) but that didn't help at all either.