\underline does not allow line breaks.
Package ulem
The text must not hidden in a macro or in curly braces.
\documentclass[10pt,a4paper]{article}
\usepackage{hyperref}
\usepackage{lmodern}
\usepackage[left=2cm,right=2cm,top=2cm,bottom=2cm]{geometry}
\usepackage[normalem]{ulem}
\newcommand\lipsumi{Lorem ipsum dolor sit amet, consectetuer
adipiscing elit. Ut purus elit, vestibulum ut, placerat ac,
adipiscing vitae, felis. Curabitur dictum gravida mauris. Nam arcu
libero, nonummy eget, consectetuer id, vulputate a, magna. Donec
vehicula augue eu neque. Pellentesque habitant morbi tristique
senectus et netus et malesuada fames ac turpis egestas. Mauris ut
leo. Cras viverra metus rhoncus sem. Nulla et lectus vestibulum urna
fringilla ultrices. Phasellus eu tellus sit amet tortor gravida
placerat. Integer sapien est, iaculis in, pretium quis, viverra ac,
nunc. Praesent eget sem vel leo ultrices bibendum. Aenean faucibus.
Morbi dolor nulla, malesuada eu, pulvinar at, mollis ac, nulla.
Curabitur auctor semper nulla. Donec varius orci eget risus. Duis
nibh mi, congue eu, accumsan eleifend, sagittis quis, diam. Duis
eget orci sit amet orci dignissim rutrum.}
\begin{document}
\begin{itemize}
\item \href{https://google.com/}{[ID1]: %
\expandafter\uline\expandafter{\lipsumi}}
\end{itemize}
\end{document}

Package soul
\documentclass[10pt,a4paper]{article}
\usepackage{hyperref}
\usepackage{lmodern}
\usepackage[left=2cm,right=2cm,top=2cm,bottom=2cm]{geometry}
\usepackage{soul}
\newcommand\lipsumi{Lorem ipsum dolor sit amet, consectetuer
adipiscing elit. Ut purus elit, vestibulum ut, placerat ac,
adipiscing vitae, felis. Curabitur dictum gravida mauris. Nam arcu
libero, nonummy eget, consectetuer id, vulputate a, magna. Donec
vehicula augue eu neque. Pellentesque habitant morbi tristique
senectus et netus et malesuada fames ac turpis egestas. Mauris ut
leo. Cras viverra metus rhoncus sem. Nulla et lectus vestibulum urna
fringilla ultrices. Phasellus eu tellus sit amet tortor gravida
placerat. Integer sapien est, iaculis in, pretium quis, viverra ac,
nunc. Praesent eget sem vel leo ultrices bibendum. Aenean faucibus.
Morbi dolor nulla, malesuada eu, pulvinar at, mollis ac, nulla.
Curabitur auctor semper nulla. Donec varius orci eget risus. Duis
nibh mi, congue eu, accumsan eleifend, sagittis quis, diam. Duis
eget orci sit amet orci dignissim rutrum.}
\begin{document}
\begin{itemize}
\item \href{https://google.com/}{[ID1]: %
\expandafter\ul\expandafter{\lipsumi}}
\end{itemize}
\end{document}

\underlinedoes not support linebreaks. If you really really need linebreaks (please think again), them thesoulpackage (AFAIR) have an alternative, that does support linebreaks. – daleif Aug 20 '15 at 11:38\underlinealready prevents the line breaks independent from the link. Ifdvipsis used (the question does not mention it), this would be another problem on top of the non-breakable\underline, which prevents line breaks. Then it would be related (but not a duplicate). – Heiko Oberdiek Aug 20 '15 at 12:03