How to write Arabic maad sign in latex?

If I understand your question correctly, you are asking for a Unicode non-breaking space (U+00A0} with a combining maddah above (U+0653).

Here is a solution in LuaLaTeX:

documentclass{article}
tracinglostchars=3
usepackage[bidi=basic, layout=sectioning.tabular, english]{babel}
usepackage{unicode-math}
usepackage{microtype}

defaultfontfeatures{ Scale=MatchLowercase, Ligatures=TeX }
babelprovide[import=ar, onchar=fonts ids]{arabic}

babelfont{rm}
          [Ligatures=Common, Scale=1.0]{Libertinus Serif}
babelfont{sf}
          [Ligatures=Common]{Libertinus Sans}
babelfont{tt}
          {Libertinus Mono}

babelfont[arabic]{rm}
          {Amiri}

setmathfont{Libertinus Math}

newcommandspacingmaddah{foreignlanguage{arabic}{^^^^00a0^^^^0653}}

begin{document}
Hello Maddah, {spacingmaddah}

Hello Faddah.  ٓ
end{document}

You can change the fonts and the default language to your liking.

It would mostly work, with small modifications, in XeLaTeX. bidi=basic would need to become bidi=default, and you would remove usepackage{microtype} and onchar=fonts ids. The disadvantage is that the document would no longer automatically switch to the Arabic font and hyphenation when it sees Arabic writing. You would need to put foreignlanguage{arabic} and begin{otherlanguage}{arabic} tags everywhere.

There are a number of ways to approach Arabic in LaTeX. Since you're using the xetex engine, you may like the arabxetex package:

documentclass[12pt]{article}
usepackage{arabxetex}
newfontfamilyarabicfont[Script=Arabic]{Amiri}
begin{document}
This is textit{maddah:} textarab[utf]{ٓ}
end{document}

(Unfortunately, the code doesn’t show up very clearly, at least not with the fonts my browser uses, where the maddah looks like a smudge on the }. To explain, between the {} I’ve simply typed U+0653, using an editor which accepts such input. You should be able to copy and paste my code despite the problem with display.)