\documentclass[class=minimal,border={5pt 15pt 10pt 15pt}]{standalone}
\usepackage[force]{feynmp-auto}
\usepackage{xcolor}
	\usepackage{amsmath}
	\usepackage{fontspec}
	\usepackage{unicode-math}
	\usepackage{microtype}

	\setmainfont{Libertinus Serif}
	\setsansfont{Libertinus Sans}
	\setmathfont{Libertinus Math}
\begin{document}
	\unitlength=0.65mm
	\begin{fmffile}{h_boson_quartic_vertex_1}
		\begin{fmfgraph*}(40,30)
						
			\fmfleftn{i}{2}
			\fmfrightn{o}{2}
			
			\fmf{dashes,tension=1}{i1,v1,i2}
%			\fmf{dashes,tension=0.75}{i2,v1}

			\fmf{dashes,tension=1}{o1,v1,o2}
%			\fmf{dashes,tension=0.75}{o2,v1}
			
%			\fmfdot{v1}
			
		    \fmflabel{$h$}{o1}
		    \fmflabel{$h$}{o2}
		    \fmflabel{$h$}{i1}
		    \fmflabel{$h$}{i2}
			
		\end{fmfgraph*}
	\end{fmffile}
\end{document}