Sorry, I just didn't see your answer. You have add 2 bytes (255 and 254) at the beginning of your unicode file for encoding. Then you can use file_str_write as usual

Here you go:
Code:
var file=file_open_write("unicodeFile.txt");
	
//add unicode encoding
file_asc_write(file,255); 
file_asc_write(file,254);
	
//writing unicode strings works with file_str_write
file_str_write(file,myUnicodeString);
	
file_close(file);