You can, the same way you pass the parameter values via the Schedule API workflow action, is how you pass the custom state values as the parameters.
Try testing your backend workflow with a hardcoded list value to verify if you have setup things properly or not for OpenAI to accept the format you provided. In my apps I create a data type called API fails to test why API calls fail in my backend workflows. I capture the details from the API call of error body, error message and error code (If you are not familiar with those values look into how to setup API calls using API connector and use of checkbox for capture errors)
If you can confirm your hard coded format of the list is correct and the API call doesn’t fail there, then you can start to focus on what is wrong with the way you are passing those values dynamically. If it is the dynamic expression it is likely that you are using Find & Replace operators rather than Format as Text Operator that will allow you to use proper JSON formatting for lists. If you are not familiar with the Format as Text Operator to properly structure JSON for a list, check it out on the forum there are loads of posts and I think Bubble docs cover this in detail as well, but not sure about the docs.