If you are looking to create leads from a web form and have them automatically added to your list in Campaign Monitor we recommend making Salesforce your source of truth. The easiest way to add leads to Salesforce is using their web to lead solution. Once in place, this will create leads in Salesforce every time someone completes the form.
We then recommend creating a Salesforce report or list view to add these newly created records to your Campaign Monitor subscriber list. When you create the report or list it is worth limiting it to records created within the last day. Then using our import wizard you can connect the report or list view and schedule it to run every day.