In Endnote X9.2, you have to pass three steps to cite a standard:
1. Click New Reference (Ctrl+N),
2. In Reference Type, select Standard,
3. Insert other information about that standard!
Note: Sometimes, an institute advises a special format for citation of its standards. For instance, ASTM suggests a specific format at www.astm.org. Look at this example:
ASTM C143 / C143M-15a, Standard Test Method for Slump of Hydraulic-Cement Concrete, ASTM International, West Conshohocken, PA, 2015