Hiya
@JI
I think
@Jessica Booker would probably have more to say here, but I've never thought it necessary to include the firm’s address or manually insert a letterhead unless the instructions specifically ask for it. The focus is normally on the content, structure, and professionalism of the letter, rather than replicating formatting details like a letterhead. If no guidance is provided, I think it's safe to start the letter directly with the recipient's name and title (if provided) or with a formal salutation (e.g., "Dear [Recipient's Name]").
Hope that helps!