If you generally discuss a conversation in a chat tool, you can refer to the conversation in your prose or in an endnote without creating a works-cited-list entry. However, if you quote directly from a single message or paraphrase its contents, create a works-cited-list entry for the message and follow the MLA format template.
To cite a message sent to one person, begin your entry by listing the sender of the message as the author. Provide a description in place of a title, and include the recipient’s name in the description. Provide the name of the chat tool in the “Title of container” slot, and then give the date of the message. Since the URL cannot be made publicly available, do not include the URL in the entry.
Smith, Sarah. Message to Ellen Wright. Slack, 10 Oct. 2017.
If you are citing a direct message sent to a group of people, include that information in the description:
Davis, John. Message to Jennifer Alonzo et al. Slack, 8 Jan. 2016.
In a chat tool like Slack, users belong to workspaces that are made up of channels. In those individual channels, users can post messages that all the members of a channel can see. If you are citing a message posted in a channel, you can note the name of the workspace and channel in the description:
Cooper, Courtney. Message posted in the #New Books channel of the Karen’s Book Club workspace. Slack, 12 July 2014.