Although I’ve set up payment arrangements with them, I owed the IRS last year, will they take my current tax return to pay off what I owe them?

Yes, if you still owe tax to the IRS, your 2018 refund will be seized to pay the debt even if you are on a payment plan.  You do not have to enter anything about it on your tax return.  The IRS knows and will take care of it automatically.  They will send you a letter a few weeks later.