The fifth SEISS grant is the last in a series of government payments worth up to £7500 made available to support eligible self employed individuals through the pandemic. There are certain criteria which you will have to meet in order to claim, details of which can be found from this link https://www.gov.uk/guidance/claim-a-grant-through-the-coronavirus-covid-19-self-employment-income-support-schemehttps://www.gov.uk/guidance/claim-a-grant-through-the-coronavirus-covid-19-self-employment-income-support-scheme
HMRC will contact you to give you a date that you can make your claim from. It will be given to you either by email, text message, letter or within the online service. Please note that you will have to claim by 30 September 2021.