Depending on the access you have, your view of the following may vary.
To connect your Experience to an opt-in email list you will start by clicking "+ Add Element" on the left hand side and then selecting the "List Opt In" element.
Next, you'll need to add the text that will appear next to the checkbox in the "List Opt In Content" here:
Once that has been done, you'll click the "+ Add Integration" button, hover over "External Integrations" and then select the integration you would like to connect it to, or set up a new integration.
Then just select the list you would like those who opt in to be added to and you're done!