Each autoresponder/ CRM does have its own limit of the list that can be displayed based on their API. That being said, not all lists can be retrieved to the dropdown menu if you have a lot of lists created in your autoresponder/CRM.
In these cases, you will need to use the custom ID feature.
To use a custom ID, you will first need to select your integration. Then, you should see the "Email Service/ CRM list" field in which you will need to select "Custom ID" instead of a list.
Finally, enter in your custom ID directly from your autoresponder and then map the field. Click "Save" in order for the changes to be reflected.
For Custom ID integrations, the integration list name will show as "nil" instead of the full list name but it will not affect the syncing.
For Aweber custom ID, you will only need to enter in the number part.