Writers Workshop was created in 1958.
Gotham Writers' Workshop was created in 1993.
Asian American Writers' Workshop was created in 1991.
Erma Bombeck Writers' Workshop was created in 2000.
That would depend totally on its meaning. If the workshop belongs to the writers or writer then you would use the apostrophe for the possessive. However, if you are saying it is a workshop for writers then the plural is correct.
The web address of the Asian American Writers Workshop is: http://aaww.org
The phone number of the Asian American Writers Workshop is: 212-494-0061.
The Jazz Workshop was created in 1957.
Development Workshop was created in 1973.
Sesame Workshop was created in 1968.
Drum Workshop was created in 1972.
The Radiophonic Workshop was created in 1975.
The Animation Workshop was created in 198#.