May 18 (Reuters) - Squarespace Inc was valued at $6.5 billion in its market debut on Wednesday, after its shares opened 4% below their reference price. Shares of the website building and hosting company opened at $48, down from the reference price of $50 a share. Reporting by Niket Nishant in Bengaluru; Editing by Vinay Dwivedi Our Standards: The Thomson Reuters Trust Principles.