Found what I was after, still unsure as to setting a Steam but this will get you most of the way:
https://portal.perforce.com/s/article/3504
When creating a new client, the only pieces of information that are
required are the name of the client and its root. The name is given
upon invocation, but the workspace root normally has to be specified
in the creation form or automatically generated by sed. However, you
can take advantage of the fact that the workspace root is set, by
default, to the directory where the client creation takes place.
If you are creating a client specification called "testme" and you
want its root to be "D:\testme_root", you do the following:
Change to the proper directory.
cd D:\testme_root
Note: This method creates a client specification
with the default client view mappings for the user.
Run the p4 client command using standard input and output.
D:\testme_root>p4 client -o testme | p4 client -i
You can then set the workspace for streams using:
D:\testme_root>p4 client -f -s -S //stream_root_path