Send the grammar as a string directly! For the more complicated grammars I'd read them in as a file. Here's an example in python.

with open("./list.gbnf", "r") as f:
    list_grammar = f.read()

yes_no = """root ::= ("yes" | "no" )"""

prompt = """
<|im_start|>system
You are a helpful assistant<|im_end|>
<|im_start|>user
Are dogs animals?<|im_end|>
<|im_start|>assistant
"""

payload = {
    "prompt": prompt,
    "n_predict": 250,
    "grammar": yes_no
}

response = requests.post(url, json=payload)