app.py

import gradio as gr

mf={
    "Heading1":"# $INP",
    "Heading2":"## $INP",
    "Heading3":"### $INP",
    "Bold":"**$INP**"
}
def upd(inp,format):
    line=f'{mf[format].replace("$INP",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("")
    btn.click(upd,[txt,format],prev)
app.launch()