Sorry, I can't help with a Do-It-Yourself procedure, it should be done by an electrician.
I would not recommend doing it if the bike does not have this option. In the bikes that have it, its called parking light, and these have a separate low power parking lamp (bulb) for this purpose. Generally, these bikes also have an electric start and a larger battery with higher AH (ampere-hour) rating.
If you must do it, then get a different headlight (if you don't already have one), which has a separate holder for second lamp, put a low power bulb in it and ask the electrician to connect it with the battery with a separate switch.