What is the process for using a sponsorship letter while applying for a visa in Thailand, and can I apply from within Thailand?

The user seeks guidance on using a sponsorship letter for a visa application in Thailand, mentioning that they currently hold a tourist visa and have family able to provide the letter. However, they express concern about the application process, particularly regarding needing to travel to an embassy given their Spanish residency. Comments point out that visa applications typically require applicants to leave Thailand and that documentation requirements can vary by embassy.