Jesus! So we're gonna depend on you the chronic smoker to defend us from a terrorist attack now??
Let me give you some advice. I work 20 years for the federal government, 15 years overseas working with the DEA, customs, military, foreign governments. Your 18, you have the ability now to land a decent job, government jobs are pretty cushy and you really don't want to fuck this up.
Number one, get clean and stay clean, they (TSA) do random testing and sometimes the observer is standing right beside you, none of this here's a vial go piss BS. They have observers, not always but you cannot count on being alone. If you get the job you will be tested often, I've seen guys test (random) 3 times in a single month. It's random, but sometimes it sure doesn't feel that way.
Two, if you persist in smoking and still decide to test you have only one chance of passing it, clean urine from a company that sells it, and they do. They test immediately after the sample is given for temperture. If it is not warm enough (the sample) it is immediately flagged as suspect and they may reschedule you and place an observer in with you. My guess is that for pre-employement there will be no observer, once hired forget that shit. I passed a physical for the FAA recently that required a urine sample and I was not clean but I had time to get the things I needed. I think I spent $50-75 bucks but it was worth it.
Any thing that is added to the sample can be detected, so you can only pass with clean urine. There is a 100 PPM threshold so you can still have THC in your system but if it's below the threshold it is returned negative. Stay away from addatives or synthetic urine.
Real urine, when purchased comes in a sterile container (bag), is place under your arm near your arm pit and tape in place. A hose goes from there to your pants where a small clip can be released to start the flow. Do not use this if being observed, the fuck'n click (when you release it) is a tell'tale sign something is up. It is kept under your arm for at least 90 minutes to warm to body heat temperture. Remeber it can only be off a few degrees from body temp or it's flagged.
If all goes well and no observer is present your home free.
But do yourself a favor, quit smoking and start a new life as a TSA agent! A good job is sometimes more important in life than smoking pot, others may argue differently but you will have years to enjoy your habit later once you have retired. If you can't stay clean don't even go in for the test. You'll waste a few years training and than get canned anyway for a failed test down the road. I have seen 50+ individuals get the boot for this, some with as much as 18 years federal service, 2 years from retirement.
And I have never heard them test for semen, there are enough genetic markers in urine that you wouldn't test for semen. Mom is just trying to scare you into quiting smoking and taking a good job, LISTEN TO HER!