You might be exempt if the minimum amount you'd have to pay for the annual premiums is more than eight percent of your household income.
You don't necessarily need an ECN to claim the exemption -- please see "Types of Coverage Exemptions" on page 3 of the IRS' 2018 Instructions for Form 8965 to see what's needed when you file your return.
When you're ready to file your return, please see the TurboTax FAQ "How do I file Form 8965 for the health coverage exemption?" for guidance on entering your exemption information. I also attach a couple of additional FAQs, just FYI.
How do I file Form 8965 for the health coverage exemption?
Can I get a health insurance penalty exemption?
What health care penalty exemptions are there for financial hardships?
To receive an ECN (Exemption Certificate Number), you would've needed to apply for insurance with the Marketplace. If you qualified for an exemption, the Marketplace would've provided with an ECN. For information on applying for health insurance, please see HealthCare.gov.