Micrososft Word creating a textbox with only top border

Solution 1:

A variation on Mr. J’s answer: insert a text box, remove the border, then insert a horizontal line by typing --- (three dashes) followed by Enter.  If there’s too much white space above the line, select the paragraph above the line and shrink it –– Ctrl+<, i.e., Ctrl+Shift+,, is a keyboard shortcut that does that.

Solution 2:

To achieve the same effect as the others have described, you can also select the first paragraph in your text box and set its top border to whatever line style you want. This option is in the Paragraph section of the Home ribbon.

Solution 3:

you can put in a plain text box, then format it to remove the border ("no line"), then insert a 1 cell table in the text box and you can set either the top or bottom border to solid or whatever