Are you referring to the CLI setting for the BSS Minimum data-rate?
If so the Unleashed command is, for example to set the minimum data-rate at 12Mbps:
ruckus(config-wlan)# bss-minrate 12
You're probably already aware but you will first need to load the WLAN you want the BSS minimum data-rate config to apply to. E.g.:
ruckus(config)# wlan ruckus2
The details are in the Unleashed CLI documenation on the Ruckus support site (you will need an active support contract to access).
Hope that helps,