Can I make a Macro to do this?

This does the trick.

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Application(wp; "WordPerfect"; Default!)

vPW = ?PaperWidth
vMR = ?MarginRight
vP = ?Position
vLen = vPW - vMR - vP

GraphicsLineCreate ()
GraphicsLineLength (vLen)
GraphicsLineHorizontalPosition (Right!)
GraphicsLineEnd (Save!)

Return

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Have the cursor at the end of the text and call the macro.

To assign a macro to a menu item, a toolbar button or a keystroke - please see Barry MacDonnell's notes at

wptoolbox.com/.../EasyPlay.html

Another way to do this with a macro is to underline tab spaces up to the margin:

See the (free) Underline2Margin macro, downloadable on my site at
https://wptoolbox.com/library/UnderLin.html

"A macro that creates an underline from the current cursor position -- i.e., after the last text character on a line -- up to the right margin to "fill" the space with a line. It then (optionally) enters a hard return [HRt] and (optionally) returns the cursor to the starting point. This allows the line to automatically expand or contract if previous text on that same line is edited.
It is useful (for example) to create a section divider that visually separates a description from following text. ..."

The download file contains 2 other (related) macros. See also the tips on the download page.