Ind. Code § 22-4-10-5
Any employer may make voluntary payments in addition to the contributions required under this article, and the same shall be credited to its experience account. The voluntary contributions shall not be used in the computation of reduced rates unless the contributions are paid prior to the expiration of one hundred twenty (120) days after the beginning of the year for which the rates are effective. The payments shall be included in the experience account as of the computation date only if they are made within thirty (30) days following the date upon which the department sends notice that the payments may be made with respect to a calendar year. The voluntary payments when accepted from an employer will not be refunded in whole or in part.
IC 22-4-10-5