Configure Slack in Jenkins using a post-initialization script (https://wiki.jenkins.io/display/JENKINS/Post-initialization+script)

configureSlack.groovy

import static com.cloudbees.plugins.credentials.CredentialsScope.GLOBAL

import com.cloudbees.plugins.credentials.domains.Domain
import com.cloudbees.plugins.credentials.SystemCredentialsProvider
import hudson.util.Secret
import org.jenkinsci.plugins.plaincredentials.impl.StringCredentialsImpl
import com.cloudbees.plugins.credentials.CredentialsStore

// Configure the global Slack token as a secret text credential
CredentialsStore credentialsStore = SystemCredentialsProvider.getInstance().getStore()
StringCredentialsImpl secretTextCredentials = new StringCredentialsImpl(GLOBAL, "slack-token", "", Secret.fromString("<YOU_SLACK_TOKEN>")) // TODO add token
credentialsStore.addCredentials(Domain.global(), secretTextCredentials)

// configure the Slack plugin
def slack = Jenkins.instance.getExtensionList('jenkins.plugins.slack.SlackNotifier$DescriptorImpl')[0]
slack.with {
  teamDomain = "<YOU_TEAM_DOMAIN>" // TODO set team domain
  tokenCredentialId = "slack-token"
  // token = ... // alternative: set plain token
  // baseUrl = ...
  // botUser = false
  // room = ...
  // sendAs = "Jenins"
}