How about this?
svn copy ^/@HEAD <Branch URL>
EDIT
As branches, tags and connecting lines are just paths and completely arbitrary in SVN, I don’t think that you can accomplish what you are trying to do without creating a bit of creativity (for example, outside the built-in - in the SVN command). To accomplish what I think you want to do, you can use a bash script (assuming Mac / Linux):
#!/bin/bash
CURRENT_URL=$(svn info | grep ^URL | cut -d" " -f2)
svn copy $CURRENT_URL "^/branches/$1"
Using: svnbranch.sh newbranch
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