Yes, talk to HR/Payroll, because it depends on the company. At my last company, I could contribute up to 50% of my paycheck. At this company, I can contribute up to 80%. Whether you can hit $18k on that depends on your salary, obviously. I just started my current job a couple of weeks ago, so I have 4 paychecks left in 2016 and my salary is high enough that I can contribute $4500/check without hitting the 80% ceiling, so I'm doing that.
Note - We're all assuming that you are in fact entitled to contribute $18k at this employer (as in, you have not contributed to a different employer 401(k) during 2016). I assume you know that it's the max for the tax year total, not per employer, but I thought I'd throw it out there just in case.