When a visitor submits a HubSpot form on one of your website pages, HubSpot will attribute the contact to your ad, as long as you've installed the HubSpot tracking code on your website and you've selected the appropriate attribution report on the Ads dashboard.
Each ad network allows you to define how to measure a conversion on your ad. These network-defined conversions will sync to HubSpot under the Network conversions field when you review metrics on your Ads dashboard.
Contacts from Facebook and LinkedIn lead ads
Both Facebook and LinkedIn allow you to create lead generation ads, which provides users a way to fill out a form directly on your ad. Any users who submit the form on your lead ad will become conversions in your ad account. If you've set up lead syncing for your Facebook page or LinkedIn ad account, these conversions will sync directly to HubSpot as contacts attributed to that ad.