As individuals, we make choices about the products we buy and the services we use. We have the (consumer) power to boycott companies who support Brexit, if we wish.
You’ll find details of pro-Brexit companies on the RemainerAction boycott list
But when you want to positively support companies who believe that Remaining in the EU is the best option for the UK, that can be harder. Search engines don’t find much.
In practice, supporting Beefeater (a Whitbread company) looked like this !