This extension is based on dunglas/mercures implementation.
To install it, for DDEV v1.23.5 or above run
ddev add-on get Rindula/ddev-mercure
For earlier versions of DDEV run
ddev get Rindula/ddev-mercure
The mercurehost will be available under mercure.${DDEV_HOSTNAME}.
So for example.ddev.site the url will be mercure.example.ddev.site.
From inside the web container, you can reach mercure under http://mercure:3000.