Please try adding help_text under sub parts as below
"sub_parts": [
{
"help_text": {
"content": "This is the help text"
}
},
{
"quick_reply_button": {
"display_order": "1",
"label": "Yes"
}
},
{
"quick_reply_button": {
"display_order": "2",
"label": "No"
}
},
{
"quick_reply_button": {
"display_order": "3",
"label": "Nope"
}
}
]