Specify the Slack channel and message text. For this:
Point to the import task, and click Edit.
Click the Next button twice, and, on the Mapping Definition page, click ChannelId.
Enter the required channel name in the box instead of "Please enter your channel name...".
Scroll down more and click Text.
Enter the text for the message in the box instead of "Please specify text of the message to send..." and click Finish.
Specify the schedule, on which the package will run automatically (See how)
If you need to create Slack notifications only for new WordPress posts, matching some condition, you can add filter conditions for the source posts. For this point to the import task and click Edit, then configure filters as described here.
If you want to use some information from the source posts in your message, you can use the expression mapping (instead of the constant mapping) for the Text field. Skyvia allows you to build expressions including source fields and using various functions and operators.