In particular (copied from the ACM guidelines), you have to send your request for a letter in support of a visa application to email@example.com including the following information
- Your name as it appears on your passport
- Your current postal mailing address
- The name of the conference you wish to attend
- Your Registration Confirmation Number
- If you have any papers accepted for the conference, please provide the title and indicate whether you are the 'sole author' or a 'co-author'.