What if I have a button that is only displayed on the list view of Salesforce. How do I use that but

Here the SF Custom Button will not have enough context to work correctly.  It does not know which items you have selected.  To do this correctly you will have to make this button a “Mass Action” on the table and include some code so that the selected records are passed as an array into your URL. 

Zach showed how to do this with Marketo Emails and I believe your situation will be very similar.