When the reference is to a work by a corporate author, use the full name of the organization as the author.

Retired officers retain access to all the university’s educational and recreational facilities (Columbia University, 1987, p. 54).

If the name is long and cumbersome and if the abbreviation is familiar or readily understandable, you may put the abbreviation within brackets in the first citation and abbreviate the name in subsequent citations.

[First citation] Though the brain may be done growing in size, it does not finish developing and maturing until the mid- to late 20s (National Institute of Mental Health [NIMH], 2020).

[Subsequent citation] Because the teen brain is still developing, teens may respond to stress differently than adults (NIMH, 2020).

The above appears in the reference list without the abbreviation:

National Institute of Mental Health. (2020) …