Square Inc. /zigman2/quotes/205989440/composite SQ +4.90% said Thursday that it would begin allowing developers to build on top of the Square Online platform. The company announced at its Unboxed developer conference that its new Snippets API function would let developers make plugins to enhance the online-shopping experience. "The ability to build extensions for Square Online was one of the most requested developer features over the past year due to the rapid growth of online selling," the company said in a blog post. Square Online has seen its number of active sellers more than double each quarter since the beginning of 2020, according to the release. The company also announced new application programming interfaces (APIs) for loyalty rewards programs and web payments. Square shares are off nearly 4% in Thursday afternoon trading. The stock has gained 169% over the past 12 months as the S&P 500 /zigman2/quotes/210599714/realtime SPX -0.04% has risen 46%.