Section 142(3) of the CGST Act, 2017 says, “every claim for refund filed by any person before, on or after the appointed day, for refund of any amount of CENVAT credit, duty, tax, interest or any other amount paid under the existing law, shall be disposed of in accordance with the provisions of existing law and any amount eventually accruing to him shall be paid in cash, notwithstanding anything to the contrary contained under the provisions of existing law other than the provisions of sub-section (2) of section 11B of the Central Excise Act, 1944 (1 of 1944)”.
So, you may file an application in the prescribed Form R, along with a copy of the challan evidencing payment of duty and a certificate from Customs regarding the duty payment towards regularisation of default under advance authorisation, and a covering letter explaining the circumstances leading to the refund claim.