I'd have to see a mockup of your document to give a better answer. But rather than hard spaces, this is a good time to use an underlined tab character immediately after your bookmark and put the tab stop wherever you want your line to end. Probably not necessary to use hard spaces or coding for that effect.

However, if you're in a table cell... You could also use a cell border.

Or, you could use a paragraph border and adjust indents.

Many ways to format a signature line without hardspace/underline formatting or a lot of underscore characters