I'm guessing you'd get more useful replies if you actually posted an example of what you wanted your key to look like. Posting a command that I cannot run without installing software is not particularly helpful. I also didn't find any particularly consistent examples with some cursory google searching. As noted above, a 25519 key is a series of random bytes. There are numerous standard ways to represent bytes in a string form. Base64 is one of those standard ways and is, for example, how 25519 keys are displayed in a WireGuard config file. Aside from base64 (or one of its variants), there's also hexadecimal strings (in which each byte is represented by two characters in the range of 0 - 9, a - f). If you post an example of what you're looking for, someone may be able to identify for you what the format is and tell you how to reproduce it. UTF8 is used to represent strings in byte form and cannot be used to represent arbitrary bytes as a string.