Right now you can't do both - that is, you can’t be covered as a dependent under your parents' plan and apply your tax credit to their premium. This is because the federally facilitated Marketplaces do not currently allow multiple tax households, in this case, you and your parents, to enroll in a plan together and apply any combined tax credits to the premium. Instead, your parents and you must file a separate application for each tax household. Each tax household will be its own policy, although you can pick the same health plan if you choose to do so. With your income at 260 percent of the federal poverty level, you will likely qualify for tax credits.
In general, you can be covered as a dependent up to age 26 on your parent's marketplace policy, but right now, due to technical limitations of the marketplace, if you want help paying for coverage, this only happens if your parents claim you as a tax dependent and together, as a tax household, you qualify for financial assistance. (45 C.F.R. § 147.120; 26 U.S.C. § 36B; CMS, Application Spotlight: Family and Household Composition Section, March 2017.)