How to use the nonebot.on_request function in nonebot

@on_request('group')
async def bot_request(session: RequestSession):
    global request_flag
    f_group= session.ctx['group_id']
    f_user= session.ctx['user_id']
    if session.ctx['sub_type']=='invite':
        if request_flag == False:
            await session.bot.send_private_msg(user_id=f_user,message=f'想邀请可可萝入群嘛？请先点击这里联系作者：\nhttps://wpa.qq.com/msgrd?v=3&uin={master}&site=qq&menu=yes')
            await session.bot.send_private_msg(user_id=master,message=f'有新的群邀请:\n群：{f_group}\n邀请人：{f_user}\n已忽略')
        else :
            try:
                await session.approve()
            except ActionFailed as e:
                print(e.retcode)
            await session.bot.send_private_msg(user_id=master,message=f'有新的群邀请:\n群：{f_group}\n邀请人：{f_user}\n已同意')
            request_flag=False