To create custom objects in SalesForce through form submissions, go to the Settings → 3rd Party Apps section of your form and select Custom Object in the Create Object dropdown of the SalesForce application.
Next, hit Customize and select the type of custom object you want to create in your SalesForce account, in the Custom Type dropdown. Here, you’ll find listed standard and custom objects of your SalesForce account. Once selected, all SalesForce fields associated to the object will appear below. Match them with you form fields to send the form data you need. Click on Save after editing and you are done.