What should the letter say? To come to Clark to study, work, or conduct research, you will need a visa document from the International Students and Scholars Office (ISSO).Comprehensive guide to writing an invitation letter for your US visa application, with free samples and tips. Edit, sign, and share letter invitation online. No need to install software, just go to DocHub, and sign up instantly and for free. Online Nonimmigrant Visa Application, Form DS-160 – Learn more about completing the DS-160. For a US tourist visa, no such letter is required, nor will such a letter if written will even be read or considered during the interview. According to the US Department of State – Bureau of Consular Affairs, letters of invitation are not a factor in issuing or denying a visitor visa. The Bureau of Immigration acts as the primary enforcement arm of the Department of Justice and the President of the Philippines.