Create Slack Application using https://api.slack.com/apps?new_app=1
Once application is created navigate to Enter OAuth & Permissions
Click Permissions under Add features and functionality section and add chat:write:bot scope. To use the optional username and icon overrides in the Slack notification service also add the chat:write.customize scope.
Scroll back to the top, click 'Install App to Workspace' button and confirm the installation.
Once installation is completed copy the OAuth token.
Finally use the OAuth token to configure the slack integration in the argocd-notifications-secret secret: