Approaching the end of the first quarter of the 21st century, the world is abuzz with the possibilities of Generative Artificial Intelligence (GenAI) writing tools such as ChatGPT. This paper argues writing prompts for GenAI tools can be designed using understandings of genre, register and grammar meaning resources from the Language strand of the Australian Curriculum: English (ACE). This paper aims to explore how educators can draw on knowledge about genre and contextual register variables to investigate the contextual information provided to GenAI writing tools to generate written texts. Specifically, the paper asks how functional grammar descriptions of context can inform the writing of prompts used by GenAI writing tools to generate texts. First, functional grammar understandings of genre and the register variables of field, tenor and mode and their connection to the ACE Language strand are explained. Next, how the genre and contextual variables can be used to create a writing prompt for the GenAI writing tool is described. After this, the GenAI writing tool ChatGPT is introduced, and a generated writing sample using the developed genre and register prompt is presented. Finally, implications for teaching grammar in an age of GenAI writing tools are canvased.