is it the default config?

You already know the answer - specify it as a proper UTF-16 string.

Unicode codepoints above U+FFFF are represented in UTF-16 using a surrogate pair, which is two 16bit codeunits acting together to represent the full Unicode codepoint value. For U+1F50E, the surrogate pair is U+D83D U+DD0E.

In Qt, a UTF-16 codeunit is represented as a QChar, so you need two QChar values, eg:

edit.setText(QString::fromWCharArray(L"\xD83D\xDD0E"));

or:

edit.setText(QString::fromStdWString(L"\xD83D\xDD0E"));

Assuming a platform where sizeof(wchar_t) is 2 and not 4.

In your example, you tried using QString::fromUtf8(), but you gave it an invalid UTF-8 string. For U+1F50E, it should have looked like this instead:

edit.setText(QString::fromUtf8("\xF0\x9F\x94\x8E"));

You can also use QString::fromUcs4() instead:

uint cp = 0x1F50E;
edit.setText(QString::fromUcs4(&cp, 1));

Most of the time, people just install a patched nerd font and then you can use glyphs from it that looks like an icon.

https://www.nerdfonts.com/

After installing one, you just go to the cheat sheet, search for the glyph that you want and copy it to your config.

Although, you could also use the image widget to place any image that you want in your bar.

Install nerd fonts also.

QString::fromUtf8("€")

Will work if the file really is handled as UTF-8. As @n.m. commented, VS requires some help from a faux-BOM to ensure this.

QString::fromUtf8("\u20AC")

\u doesn't make sense in a byte string literal. You could spell it using \x byte escapes for the UTF-8 encoded version:

QString::fromUtf8("\xE2\x82\xAC")

Or use a wide string literal:

QString::fromWCharArray(L"\u20AC")