Going with your information, I assume you are currently on an International Experience Canada - Young Professional permit? If this is correct (or if you have a different work permit that specifies your employer) you can claim the extra 50pts. if:
- You have been working for this employer for at least 12 months
- The job offer is for a NOC 0, A or B position
- The job offer is for the same NOC position that you have been working under in the last 12 months.
- The job offer must remain valid for at least 12 months after you obtain your PR
- The job offer must be for a full-time/permanent position (at least 30hrs/week)
If you indeed can claim points, you will simply need your offer of employment and upload it after you're given an ITA. Your employer does not need to do anything, other than giving you a job offer and signing it.
For expiration date, I would recommend setting it to something like March 1, 2020 (which is what I did) which means that you will easily meet the 12-month threshold as PRs are usually obtained within 6 months. I don't know if it's okay if the offer simply mentions you will remain employed for 1 year after obtaining a PR (someone else should confirm this).
If you have any other questions, feel free to ask - I traversed essentially the same path as you with my PR application, and so far have not yet received any feedback about issues with my job offer.