Hi @Your_Small_Business and welcome to Activepieces!
You need the Ninja Forms Webhook Addon in order to be able to use Webhooks there. Once you install it, you can get the webhook url as @Bram explained on Contact Forms Wordpress Submission to Google Sheets Please Help and paste it into Ninja Forms Webhooks.
Then you can test the webhook and use the data on Activepieces.
Does this make sense?