Contact your ward clerk to get that.
If you do not know what your home ward is, you can use the
Meetinghouse Locator found in the Resources menu (waffle icon, or grid of 9 square dots icon) to the left of the Sign In button on the Church website to find your home ward. Then enter your address in the search box. The clerk may need to move your record into the ward to see your membership record number (MRN).