To integrate SaaS Alerts into Slack you will want to either create a new channel or use an existing one.
Within the Slack UI right click on the channel you wish to use and select Open Channel Details
Then select the Integrations tab and click the Send emails to this channel option
This will then generate an email address (which you may customize) to use for alerts
Now within the SaaS Alerts UI navigate to the Settings tab and scroll down to PSA & Email, click the green "+" symbol and add the address that was generated in the above step
You have now integrated SaaS Alerts into Slack!