I think your questions are written too simple to answer here. In my view, an answer to these questions is a complete implementation of the functionality. It starts with reading the documentation of the Salesforce API, which explains what kind of calls are expected and what their structure should be. You can then implement this in Mendix based on this. You often have to deal with Custom headers or a redirect. I would ask your questions more specifically, perhaps people with experience in this field (Salesforce-Mendix integrations) can answer your questions more easily.