Md. Code, Tax-Gen. § 7.5-201

(a) Each person that, in a calendar year, has annual gross revenues derived from digital advertising services in the State of at least $1,000,000 shall complete, under oath, and file with the Comptroller a return, on or before April 15 of the next year. (b) (1) Each person that reasonably expects the person’s annual gross revenues derived from digital advertising services in the State to exceed $1,000,000 shall complete, under oath, and file with the Comptroller a declaration of estimated tax, on or before April 15 of that year. (2) A person required under paragraph (1) of this subsection to file a declaration of estimated tax for a taxable year shall complete and file with the Comptroller a quarterly estimated tax return on or before June 15, September 15, and December 15 of that year. (c) A person required to file a return under this section shall file with the return an attachment that states any information that the Comptroller requires to determine annual gross revenues derived from digital advertising services in the State. (d) For periods beginning after December 31, 2026, a person shall file a digital advertising gross revenues tax return electronically.