When creating a landing page, to add a form, go to Pre-designed blocks:

Choose Forms and drag and drop the design that you’d like to choose:

When you drag and drop the form, you will see the settings on the left to create a new list or use a previously created list (the previously created list has the fields you’d like to include while the new list created only has the fields shown in the form chosen):

Once you finish configuring the form settings, click on apply changes:

In case you missed the settings or want to access them again, click on the gear icon:

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