app.py

import streamlit as st
import streamlit.components.v1 as components

st.chat_input("biggie")
default_chat_input_value = "Default Value"
js = f"""
    <script>
        function insertText(dummy_var_to_force_repeat_execution) {{
            var chatInput = parent.document.querySelector('textarea[data-testid="stChatInput"]');
            var nativeInputValueSetter = Object.getOwnPropertyDescriptor(window.HTMLTextAreaElement.prototype, "value").set;
            nativeInputValueSetter.call(chatInput, "{default_chat_input_value}");
            var event = new Event('input', {{ bubbles: true}});
            chatInput.dispatchEvent(event);
        }}
        insertText({len(st.session_state.messages)});
    </script>
    """
components(js)