Step 1: Log in to Hubspot
On the Run form builder, click to add destination, and after you select Hubspot, click the button to login. After you've successfully logged in, click the Save button. Confirm that it's connecting correctly under Status after you save.
Step 2: Select Hubspot as the destination on ALL forms you plan to use
On the form builder, you must click on the Enable checkbox to the left of the destination for all forms you want to use this destination for on the site.
Custom Fields (custom Properties)
In order to add custom fields (called custom Properties on Hubspot) to your form, you'll first need to add those fields on Hubspot.
Use these settings for your custom Property:
Object type: Contact
Group: Contact information
Field type: Single-line text or Multi-line text
Then, open your Run form builder, click Add Form Field, and add a Custom Field (like 'Text' for example). Under the Hubspot custom fields column, Run will pull down a list of your custom fields from Hubspot to select from.