app.py

import gradio as gr

mf={
    "Heading1":"#",
    "Heading2":"##",
    "Heading3":"###",
}

def upd(inp,format):
    line=mf[format]+inp
    return line
with gr.Blocks() as app:
    format=gr.Dropdown(label="Line Format",choices=[m for m in list(mf.keys())])
    txt=gr.Textbox()
    btn=gr.Button()
    prev=gr.Markdown("# test")
    btn.click(upd,[txt,format],prev)
app.launch()