The guidance for the newitem says:
Specifies the name of the destination file or folder or the parent
folder for the destination. If newitem already exists and is a Team
Foundation version control server folder, Team Foundation creates the
branched items within it. Otherwise, newitem specifies the name of the
destination file or folder. Conflicts can occur during check-in if the
destination already exists.
Might be better been explicit and using the following command:
D:\Dev...>tf branch D:\dev\path\to\proj $/MyTeamProject/branch_name