This method is ONLY if you want simplified targeting: If you would simply be able to target people who have visited your landing page, without the complexity, here is how to create an audience of people who have viewed your landing page, this can be any page of your lead capture sites.
Advanced Custom Audiences
To Create an audience for people who visit your Landing Page only, this is what your Custom Audience should look like:
To create an audience to target people who opted into page 1 but dropped off on Page 2, this is what your Custom Audience should look like:
To Create an audience to track completed opt ins, this is what your custom audience should look like. You can use this audience to EXCLUDE Opt in's from your target audience and save costs on your advertising (you will no longer be showing your ad to people who have opted in).
This is a screenshot of Ads Manager, where you are selecting your custom audience and you can select to exclude the audience from your targeting.