i am experienced in substituion for employment ua's have done this 3 times and always passed let me tell you what i did:

my employer gave me the paperwork and the lab adress etc. etc.
on the way to the lab i take my synthetic urine and the bottle its in and microwave it at a gas station then apply the temp stip to make sure its around 100 degrees F

simply put that in my sock under the tounge of my shoe... some have made me empty my pockets and gave me a brief pat down but have check my shoes at all, other's didnt even do that simply told me to go in their bathroom and pee

anyway its easy step in the bathroom and while you take a piss in the toilet i put my fake piss in the cup so it sounds legit the temp is always about right and i was good to go

in my experience pre-employment screens are very easy to substitute and if you can remain calm everything should go well