Yes, you will use the EIN if you are "in fact" required to file a SCH C for 2018. I assume the business is a sole proprietorship or single member LLC. If your business was not "open for business" in 2018, then you will not be filing a SCH C with your 2018 tax return at all. If you opened in 2019 you'll deal with this next year on the 2019 tax return.
Expenses incurred prior to being "open for business" are start-up expenses. Those start up expenses are claimed in the first year the business is open. It does not matter in what year those start up expenses were incurred either.
