There is no credential field for this action type so if you have chosen Slack for the action, there is only one additional field for Webhook URL that needs to be completed. This field needs to include a valid URL. Documentation on how to obtain your webhook URL is provided through Slack here.
If you chose Slack as your action, there is only one field available to you which is required before moving to the next step. This field is a custom text input field where you can type and/or use the chips for creating a custom Slack message as the action of your integration.