I don't ever use it, but it looks like you can use the MS Word VBO action 'Replace Highlighted Text' in order to bold text. You basically just replace the text with the same text but give a True value to 'new bold'. I don't see any existing action that will underline text.
Probably the better way to do this is to take the code stage inside of 'Replace Highlighted Text', create your own object/action, add in what inputs you want, etc.
For example, I imagine the code 'r.Bold=new_bold' from the Replace Hightlighted Text code stage can be adapted for underline. Probably need to check the Microsoft docs, but I bet 'r.Underline=True' or maybe 'r.Underline=1' will work. Of course, you'd want to take that True/1 as an input.
Dave Morris, 3Ci at Southern Company