SQL exec command using a string parameter

I'm trying to call a system command within a procedure that also carries parameters. I get the following error:

Procedure expects parameter 'command_string' of type 'varchar'

Code:

ALTER PROCEDURE [dbo].[AddMSTRUser] 
    @LoginID nvarchar(120), 
    @Country nvarchar(120), 
    @FullName nvarchar(120)
AS
    DECLARE @cmd varchar(max);

    SET @cmd = CONCAT('MASysMgr.exe -w C:\User.smw -l "C:\cache.txt" -p "LoginID=' , @LoginID , '" "Country=',@Country,'" "FullName=',@FullName,'"')

    EXECUTE master..xp_cmdshell @cmd;

This is how I call it:

EXEC AddMSTRUser @LoginID = 'Ugurcan.Kaya', @Country = 'VHQ', @FullName = 'Ugurcan Kaya'