You're seeing just the function add, go back to Slack.Web.Reactions module for more information.
Link to this function

add(name, optional_params \\ %{})

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Adds a reaction to an item.

Required Params

  • name - Reaction (emoji) name. ex: thumbsup

Optional Params

  • channel - Channel where the message to add reaction to was posted.
  • file - File to add reaction to.
  • file_comment - File comment to add reaction to.
  • timestamp - Timestamp of the message to add reaction to.

Errors the API can return:

  • already_reacted - The specified item already has the user/reaction combination.
  • bad_timestamp - Value passed for timestamp was invalid.
  • file_comment_not_found - File comment specified by file_comment does not exist.
  • file_not_found - File specified by file does not exist.
  • invalid_name - Value passed for name was invalid.
  • message_not_found - Message specified by channel and timestamp does not exist.
  • no_item_specified - file, file_comment, or combination of channel and timestamp was not specified.
  • too_many_emoji - The limit for distinct reactions (i.e emoji) on the item has been reached.
  • too_many_reactions - The limit for reactions a person may add to the item has been reached.