Freeform ReturnURL to same page I'd like my form to return to the same page with a param ?success. 

The page slug is dynamic though so will change depending on what form you are on.

/dynamic-url?success

{% set entrySlug = craft.request.lastSegment %}                         


                                    {{ craft.freeform.form("landingPage", {
                                        id: "form-landing",
                                        useRequiredAttribute: true,
                                        returnUrl: "{{ entrySlug }}?success"
                                    }).render() }}


Obviously if i just use returnUrl: "?success" it takes me back to the homepage.

The idea of this is so I can trigger facebook events upon form completion, so open to any better approaches.

Thanks
Have you tried using the {{ craft.request.url }} variable in your returnUrl setting?

Also you should be able to format it like so:

returnUrl: craft.request.url ~ "?success"


More details about the craft.request model can be found here: https://craftcms.com/docs/templating/craft.request