Depends on the manufacturer.
On Classic Roadsters tops, there is a sleeve about an inch wide sewn across the full width of the top. They provided (in early years) an aluminum bar, (in later years) a flexible plastic bar that slid into the top sleeve. Once in the sleeve, the roof folded over the top of the frame and the bar was inserted in to the slot on the leading edge of the windshield frame.
Other manufacatuers had series of snaps all along the front of the roof. The roof came over the top of the windwhield frame and attached to snaps screwed into the underside of the frame’s leading edge.
Former Owner of a 1981 Classic Roadsters Ltd. Duchess (VW)
2005 Intermeccanica Roadster
If you own a TDr and are not in the Registry, please go to https://tdreplica.com/forums/topic/mg-td-replica-registry/ and register (you need to copy and paste the link)