This is a deprecated feature but still works for simple testing. I used it mainly to test importing of configurations in a new clean environment.
To create a new partition, you must put the form name at the end of the URL.
https://usnconeboxax1aos.cloud.onebox.dynamics.com/?f=partitionadministration
Create a new one and save it.
To login to the partition, you must do a similar URL parameter by adding the partition name.
https://usnconeboxax1aos.cloud.onebox.dynamics.com/?prt=newPartition
Note, that the partition have their own data through out. Companies and security must be set up individually.