I am not a Salesforce expert, but checking this video here: Simple Web to Lead in Salesforce - YouTube , it looks like you will need to setup several fields in the HTTP Post for Campaign Builder.
The Salesforce form has several Hidden and Text fields that need to be mapped accordingly. The hidden fields can have a fixed value set in the HTTP Post, the text fields can have the Contact Merge Fields.
If you tried setting that up, and it still fails, send pictures of what you got.
If none of this work, then you will need a simple script that does a cURL Post to Salesforce instead.