Excel 365 - how do I add an underscore just above the "$ 7,709.87"?

My question is so basic, I'm ashamed to ask, but I'm asking anyway. I have a spreadsheet that has quite a few columns, all with totals at the bottom.
Some of the columns aren't as long as others, but I want the "Total Costs" to all be in the same row. Here is an example of a short column. How do I add an underscore just above "Total Costs" and $7,709.87" ? I used to be able to find it in older versions of Excel, but am not finding it in this new version. TIA