EDIT: MUST register first - application website below
"Immigration officials can be found on the 2nd floor of Central Festival Samui from 11 am to 4:30 pm from September 21 to 24. Those who want to use the service must first register via a QR code or by contacting Surat Thani Immigration directly on their Facebook page. The mobile service will also be made available in preparation for the end of the "Visa Amnesty" on September 26th."
Website: [members only]/
Facebook: [members only]
APPLICATION: [members only]
TLDR : Answer Summary
From September 21 to 24, the Surat Thani Immigration office will provide mobile services at Central Festival Samui. Registration is mandatory and can be done by scanning a QR code or contacting Surat Thani Immigration via their Facebook page. This service is in preparation for the end of the Visa Amnesty on September 26.