documentclass{article}

usepackage[most]{tcolorbox}

newtcolorbox[auto counter
%,number within=section
]{pabox}[2][]{%
colback=red!5!white,
colbacktitle=red,
coltitle=red!70!black,
colframe=red!75!black,
fonttitle=bfseries,
detach title,
before upper={tcbtitlequad},
title=Problem.~thetcbcounter: #2,#1}

begin{document}

begin{pabox}[]{}
This is a textbf{tcolorbox}.
end{pabox}

end{document}