Do I have to file form 5472 for "additional paid-in capital"?

Yes you should be reporting the capital contributions. Under the old Form 5472 rules, it's true that only items that impacted taxable income would be reportable transactions. So, a capital contribution by you to the corporation would not be reportable, unless the equity contribution was somehow below or above a fair value contribution in exchange for services that you might provide for the corporation - essentially an imputed reportable transactions.

When the IRS changed the Form 5472 rules to require non-U.S. owned single member LLC's, they expanded the reportable transaction definition to include virtually everything.

The term “transaction” is defined in Treas. Regs. Section 1.482-1(i)(7) to include any sale, assignment, lease, license, loan, advance, contribution or other transfer of any interest in or a right to use any property or money, as well as the performance of any services for the benefit of, or
on behalf of, another taxpayer.

So, for example, contributions and distributions would be considered reportable transactions with respect to such entities. These amounts can be reported on Lines 12 and 25 with an explanatory footnote that clarities the amounts are capital contributions and not amounts that impact taxable income.