Sometimes on social media I'll get random people sending me messages like this:
Hi
Hi can you help me?
Ultimately, what question do you think someone's going to ask you in response? Almost certainly:
What do you need help with?
I personally won't say yes or no until I know what your problem even is; I'm not going to commit to your problem with a "yes" without full context. Some people won't even respond to you if you don't provide full context then and there.
But there's a larger problem here; if you can easily predict what someone will say next, why don't you just answer their next question immediately instead of waiting for them to send it? People won't usually respond to your message immediately, and you'll have to wait for them to send their response. Instead, just be direct with what you want and ask it in a single message to save everyone's time.
If you're saying "hello" to just determine if someone is there, you can take the time to put a few more words into your message to increase your chances of actually getting a helpful response. If you think someone might not be there and you provide little context to your issue, chances are they won't even know what you're asking about and the context will be necessary. If they did help someone else recently though, chances are they will respond to you and you just need to be patient.
If you still want to say "hello" or include formalities, do it at the beginning of your message:
Hello! I was wondering if you could help me with (insert very specific information here)