As noted, you could utilize RLS to facilitate this but it would take a bit more configuration (adding users to a table that mapped users to brand): https://azure.microsoft.com/en-us/documentation/articles/power-bi-embedded-app-token-flow/
What you are likely looking for is passing a filter via URL. In this case, brand would be in the filters pane, not a visible filter/slicer. See this link and information here: https://azure.microsoft.com/en-us/documentation/articles/power-bi-embedded-iframe/
Filter a report
You can filter an embedded report using a URL syntax. To do this, you add a query string parameter to your iFrame src url with the filter specified. You can Filter by a value and Hide the Filter Pane.