Post to slack webhooks from salesforce process builder

SlackMessage.class

public with sharing class SlackMessage {
    
    public class Params {
        @InvocableVariable(label='Message' required=true)
        public String msg;
    }
    
    @InvocableMethod(
        label='Send a message to Slack'
        description='Send the Message argument as text to a Slack webhook endpoint.'
    )
    public static void sendSlackMessage(List<Params> ps){
        Params p = ps[0];
        Map<String, String> jsonMessage = new Map<String,String>();
        
        // Set the message text.
        jsonMessage.put('text', p.msg);
        
        // Set slack sending user info:
        jsonMessage.put('username', 'salesforce-bot');
        jsonMessage.put('icon_emoji', ':envelope:');
        
        //jsonMessage.put('channel', '#channel-name'); // to send to a channel other than the default
        //jsonMessage.put('channel', '@user'); // to message a specific user
        
        slackMessageCallout(JSON.serialize(jsonMessage));
    }
    
    @future(callout=true)
    private static void slackMessageCallout(String body) {
        HttpRequest req = new HttpRequest();
        req.setBody(body);
        // TODO: Slack will give you this endpoint url when you setup a webhook for your team
        req.setEndpoint('https://hooks.slack.com/services/...');
        req.setMethod('POST');

        Http http = new Http();
        HTTPResponse res = http.send(req);
        if (res.getStatusCode() >= 400) {
            System.debug(res.getBody());
        }
    }
}