What nurse said, but, know the legality of where you are taking the drugs and how much trouble you could be in if caught. Is it worth it?
They do swabs randomly and to be honest, I’m not sure what they test for regularly these days. I know opiates, cocaine, meth etc is standard but I’m not sure if they would pick up benzos. If it’s an unknown substance they are wary of they will be able to determine what it is whilst detaining you.
I’d also probably use a serum bottle rather than a shampoo bottle. I regularly carry face serums/acids with me when travelling, they come in dark bottles with droppers. If you look up “the ordinary serums” you’ll see what I use. An excellent brand for keeping your skin youthful and clean!