According to a statement from AmericanHort, the Departments of Homeland Security and Labor plan to release a rule on May 8, 2019 that will kick off an application process for an additional 30,000 visas for fiscal 2019. The visas will be limited to returning workers who have received an H-2B visa in one or more of the last three fiscal years.
Starting May 8, employers who have a valid Department of Labor temporary labor certification for an April 1 start date can apply to the Department of Homeland Security - U.S. Citizenship and Immigration Services. Employers can do this without re-advertising or going back through the DOL process. Applications have to be submitted through May 14.
Per AmericanHort, employers also have to submit an attestation form that says that their business, without H-2B workers, will suffer "irreparable harm" and/or suffer permanent and severe financial loss. The form is available from the Department of Labor here. After May 14, employers will have to re-advertise jobs and file a new labor certification application with the DOL.
A copy of the rule can be found here.