How to grant access to SharePoint subsite with no Owners

Hello!

Can someone advise how I can add a user to a SharePoint site with no Owners/Members or Admins?

It is a Classic SharePoint subsite so I cannot modify any settings using the SharePoint Admin Centre so I need to find a script or a method of some sort to allow me to add a user account to the site to then be able to review it/decommission it etc.?