Control code is designed and implemented for industrial automation applications that manage power plants, petrochemical processes, or steel production. Popular large language models (LLM) can synthesize low-level control code in the Structured Text programming notation according to the standard IEC 61131-3, but are not aware of proprietary control code function block libraries, which are often used in practice. To automate control logic implementation tasks, we proposed a retrieval-augmented control code generation method that can integrate such function blocks into the generated code. With this method control engineers can benefit from the code generation capabilities of LLMs, re-use proprietary and well-tested function blocks, and speed up typical programming tasks significantly. We have evaluated the method using a prototypical implementation based on GPT-4, LangChain, OpenPLC, and the open-source OSCAT function block library. In several spot sample tests, we successfully generated IEC 61131-3 ST code that integrated the desired function blocks, could be compiled, and validated through simulations.