I was talking with my japanese penpal and he wrote to me 精一杯頑張ってきますよ! after I wrote him おはようございます！お仕事頑張ってきてね！ because he told me he was going to work. I don't know if there's an answer or not. In English of course I would not respond as there's no answer to "I'll do my best". But as we are talking in Japanese and not English, I would like to know if there's a way to respond or not.
Usually I'd say it depends on the person's way of speaking, but here I feel like any reply would be "too much". At best, you can reply with a smiling face and keep it simple.
Other than that, if you really want to talk to him/her, you can change the topic, even though after this kind of conversation where there's not really much going on, it might feel like you're trying too hard to talk to him/her, and it'd end up being weird.
Or you can try to make a "joke" like "Lend me some motivation haha" and hopefully (s)he will reply with a more creative message...
But to be honest I feel like this kind of things is common to every language.
Edit: Now since some time has passed maybe you could send お疲れ様です as suggested (assuming (s)he has finished work) but that's not really part of the question since it'd be a completely different conversation