How do I pass field values that contain & as query parameters?

When passing query parameters like where in an endpoint URL, you need to URL-encode the value. Thus, & has to be passed in its URL-encoded form, which is %26. Example request to retrieve a client with name Ferdinand & Associates using the client list endpoint:

{{baseUrl}}/client?where=name%3D'Ferdinand%20%26%20Associates'

Check out the API documentation for more details on parameters.