How can I write the symbol t̬ in TeX?

I add a minimal working example for your request.

\documentclass[a4paper,12pt]{article}
\usepackage{tipa}
\usepackage{ifxetex}<------from https://tex.stackexchange.com/questions/464082/xelatex-font-shape-t3-lmr-m-n-undefinedfont-using-t3-cmr-m-n-insteadfont
\ifxetex
  \usepackage{substitutefont}
  \substitutefont{T3}{\rmdefault}{cmr}
\fi

\begin{document}
\textsubwedge{t}, \textsubcircum{t}
\end{document}

I add minimal working example using OpTeX:

\fontfam[libertine]
symbol t̬  
\bye

The main point is: use Unicode font which supports these combining diacritical marks. If this is true then you can type natural t̬ if you know how to do it in your editor and keyboard. Or you can define a macro \def\t{t̬} and use \t (the plain TeX old \t is re-defined in such case but we don't use it).