Is it possible to post to chat.postMessage as any

I'm building a Slack integration that is intended to modify some text and then post it to a Slack channel as though the user who triggered the command had said it.

e.g. /makeFace disapproval

@Ben 3:45pm ಠ_ಠ

I ask for the client permission scope, which adds the chat:write:user permission. But when I hit the chat.postMessage endpoint, it only seems to allow you to post as the user who added the integration because the token it returns seems to be individuated for that user.

I know that giphy, for instance, sends its gif messages as though you are the originator, but I can't find out how they manage it. Is there any documentation for sending messages as other members of the team?

There are 2 ways to achieve this:

A. Overwriting username and icon

When you send a message with chat.postMessage it is possible to set a user name with the property username. The message will then appear as being send by that user (same for icon with icon_url).

However, this is not meant to impersonate real users, so even if you use the same username and icon as the real user the message will have the app tag, so that they can be distinguished from a real user.

Here is an example how it looks like (from a gamer Slack about flying and killing space ships):

But depending on what your requirements are that might work for you.

If you want to use it make sure to also set the as_user property to false (yes, really) and it will not work with a bot token, only with a user token.

See here for more details on how it works.

This also works for the legacy version of Incoming Webhooks, not with the current version of incoming webhooks though. (You can still get the legacy version, see this answer)

B. Having all tokens

Another approach is to always use the token from the respective user for sending the message. In combination with as_user = true messages sent by your app will look exactly as if they would come from the respective user (no APP tag).

To make that happen your app would need to collect tokens from all users on your workspace and store them for later use. This can be done by asking every user to install your app (called adding a "configuration") through the Oauth process (same you use to install your app to a workspace), which allows your app to collect and store those tokens for later use.