Velcro type strap. Unique in that it can attach to itself anywhere. Has rubber line on it. I have received these on various bicycle bags. : r/whatisthisthing
VELCRO Brand 15 ft. x 2 in. Industrial Strength Tape 90197 - The Home Depot
VELCRO® Brand Self-Engaging Hook and Loop
VELCRO® Brand OMNI-TAPE®
Auto-agrippant Omni-Tape™ de marque VELCRO® 2 en 1 - noir - 25mm x 25m - by-pixcl