You can leverage the ngrok-python library to embed the ngrok agent in to FastAPI Applications.
ngrok-python and FastAPI libraries used in this example are sensitive to versions. Use of Virtual Environments is highly recommended
# requirements.txt
ngrok==1.4.0
uvicorn==0.29.0
fastapi==0.111.0
loguru==0.7.2

pip install -r requirements.txt

# main.py
from contextlib import asynccontextmanager
from os import getenv

import ngrok
import uvicorn
from fastapi import FastAPI
from loguru import logger

NGROK_AUTH_TOKEN = getenv("NGROK_AUTH_TOKEN", "")
APPLICATION_PORT = 5000

@asynccontextmanager
async def lifespan(app: FastAPI):
    logger.info("Setting up Ngrok Endpoint")
    ngrok.set_auth_token(NGROK_AUTH_TOKEN)
    ngrok.forward(
        addr=APPLICATION_PORT,
    )
    yield
    logger.info("Tearing Down Ngrok Endpoint")
    ngrok.disconnect()


app = FastAPI(lifespan=lifespan)

@app.get("/")
async def root():
    return {"message": "Hello World"}

if __name__ == "__main__":
    uvicorn.run("main:app", host="127.0.0.1", port=APPLICATION_PORT, reload=True)