Create a new tracking pixel and declare where it runs. Send exactly one of allowed_origins, a list of the sites the pixel runs on, or allow_any_origin: true, for a pixel that runs on any site, such as one pixel shared across many client sites. Sending neither, an empty list, or both returns a 400. Everything else matches v1: pass only a title for the defaults, or include params and custom_fields. Name the target project with a project_id field.