GST Inward credit

we have made adv pymt for online Advt agencies in the of March'18 i.e. FY-17-18 & forgot to book the invoice as well as to claim the credit of GST charged. even the same has been not mentioned in GST return for the month of March.'18

please guide us in this connection, whether we can book the bill in March & get the credit while calculation of GST payable for the month may'18

or what to do to claim the credit of GST paid in the month of March