discord.py get message using id code example

Example 1: discord.py message user

if message.content == "dm":
        await message.channel.send("Dming user")
        dm = await message.author.create_dm()  # Creates a dm channel with the user
        await dm.send("What you want to send")  # Sends the user the message

Example 2: discord.py get message text

When getting a message, you're going to need an abc.Messageable object - essentially an object where you can send a message in, for example a text channel, a DM etc.

Example:
@bot.command()
async def getmsg(ctx, msgID: int): # yes, you can do msg: discord.Message
                                   # but for the purposes of this, i'm using an int

    msg = await ctx.fetch_message(msgID) # you now have the message object from the id
                                         # ctx.fetch_message gets it from the channel
                                         # the command was executed in


###################################################

@bot.command()
async def getmsg(ctx, channel: discord.TextChannel, member: discord.Member):
    msg = discord.utils.get(await channel.history(limit=100).flatten(), author=member)
    # this gets the most recent message from a specified member in the past 100 messages
    # in a certain text channel - just an idea of how to use its versatility