Welcome to the community, @Jenny_Simon!
Are you referring to sending someone a direct message in Slack? As far as I know, Airtable doesn’t support sending direct messages in Slack. I believe that Airtable only supports sending messages to Slack channels.
Integromat has 100% full support for Slack — scroll down to the bottom of this page to see all the parts of Slack that are supported:
I am a professional Airtable consultant and a Registered Integromat Partner, and the Integromat links contain my personal referral code.
If you have a budget for your project and need to hire an expert to set this up for you, feel free to contact me through my website at scottworld.com.