\documentclass[]{beamer}
\usepackage{geometry}
\geometry{landscape,a5paper,left=1mm,right=16mm,top=1mm,bottom=0cm}
\usepackage{tcolorbox}
\tcbuselibrary{skins,hooks}
\usepackage{tikz}
\usetikzlibrary{intersections,spy,matrix}
\newcommand{\BeginBorder}[1]{\parbox[]{#1\linewidth}{\begin{tcolorbox}[enhanced,arc=0.5mm,boxrule=0.5mm,frame hidden,colback=white,borderline={0.5pt}{0mm}{black,dashed},left=0.05mm,right=0.05mm,top=0.25mm,bottom=0.25mm]}
\newcommand{\EndBorder}{\end{tcolorbox}}}
\begin{document}
\parbox[]{0.57\linewidth}
{
\begin{tcolorbox}[enhanced,arc=0.5mm,boxrule=0.5mm,frame hidden,colback=blue!4,borderline={0.5pt}{0mm}{black,dashed},left=0.25mm,right=0.25mm,top=0.25mm,bottom=0.25mm]
Some text Some text Some text Some text Some text Some text Some text Some text Some text Some text Some text Some text Some text Some text Some text Some text Some text Some text Some text Some text Some text Some text Some text Some text Some text Some text Some text Some text Some text
\end{tcolorbox}
}
\end{document}