David, if you need a single letter b underlined, and are meanwhile asking how to type a Hebrew diacritic, I suspect you actually need the underlined b for transliterating Hebrew, is that correct? Using the underline style to indicate a horizontal stroke below a letter is not the best method to achieve that. In Unicode, a number of precomposed characters with a stroke (macron) underneath are defined, e.g. ḇ (U+1E07), ḏ (U+1E0F), and several others. There are also combining diacritics U+0331 (COMBINING MACRON BELOW) and U+0332 (COMBINING LOW LINE). Combined with b they yield ḇ and b̲.
The fonts you use in Dorico may or may not have these glyphs on board. I don’t think Academico has. If the font you’re viewing this forum in lacks them, you probably see boxes or questions marks now.
So, in the end, for your first question a Unicode character selection utility may also be the answer.