Here is how I have my DIY style setup:

Water is dropping where the coin is, I put the coin there with the hope that it can focus on the coin but it couldn't. I am on AF-S , S and use AF-ON button to focus and shutter to take the picture, it was never able to focus so didn't even fire one shot. Very frustrating. How do I get it to focus?

P.S: I don't have friends or assistant to help me for example with holding his finger in there until I focus on his finger, etc...

Also please per my setup picture, if you have suggestions to improve the setup please tell me.