When executing a Job in designer, simply change the default value in the Execution Properties window. The default value can be updated in the DSConfig.txt file (located in %LINK_DIR%\Bin).
Unfortunately, it is not possible to update the Monitor Sample rate when executing a job via the Web Administrator. A common workaround is to export the Execution Command (creating both a .bat and .txt file in the %LINK_DIR%\log directory).
In the .txt file, you will then be able to update the -r parameter corresponding to the Monitor Sample Rate.
Or you go to your job in the Designer, rightclick it and select properties. There you will find in different tabs all the things you can change the defaults for. You can change the default trace options, the …, or the monitor sample rate. Once saved, it does not matter how you start the job, via Designer, export execution command, webadmin,… it will always have that default. Certainly, you can change the values still - its just the defaults you modifed with that.