To gain access to this SEP in the federally run marketplace (HealthCare.gov), you just have to attest to the fact that your income is in the eligible range.
If the income information the government has on file doesn’t match the income the applicant has projected, the exchange may request proof of the projected income to ensure that it is actually no more than 150% of the poverty level.
If an applicant does not provide proof, the federally run marketplace can terminate the premium tax credit that’s being paid on the enrollee’s behalf, meaning that the person would then be paying full-price for their coverage (or will simply find coverage terminated when they fail to pay the full premium in the first month it’s required).