Label placement on a tikzcd "to path" arrow

Question

I am using to path in order to have a better control of an arrow in a tikzcd diagram. However, I am having trouble to placement the label.

With this code

\documentclass{article}
\usepackage{tikz-cd}

\begin{document}
\begin{tikzcd}[labels=description, column sep=3cm]
  A\arrow["a+b",sloped,rounded corners,
  to path={
    -| ([yshift=-1.5cm,xshift=-.7cm]\tikztostart.west)
    -| ([xshift=.7cm]\tikztotarget.east)\tikztonodes
    -- (\tikztotarget)} ]{r}
  & B \\ C &  D
\end{tikzcd}
\end{document}

I get this

Moving the \tikztonode instruction to the first segment:

\documentclass{article}
\usepackage{tikz-cd}

\begin{document}
\begin{tikzcd}[labels=description, column sep=3cm]
  A\arrow["a+b",sloped,rounded corners,
  to path={
    -| ([yshift=-1.5cm,xshift=-.7cm]\tikztostart.west)\tikztonodes
    -| ([xshift=.7cm]\tikztotarget.east)
    -- (\tikztotarget)} ]{r}
  & B \\ C &  D
\end{tikzcd}
\end{document}

I get this

Finally, in the last segment:

\documentclass{article}
\usepackage{tikz-cd}

\begin{document}
\begin{tikzcd}[labels=description, column sep=3cm]
  A\arrow["a+b",sloped,rounded corners,
  to path={
    -| ([yshift=-1.5cm,xshift=-.7cm]\tikztostart.west)
    -| ([xshift=.7cm]\tikztotarget.east)
    -- (\tikztotarget)\tikztonodes} ]{r}
  & B \\ C &  D
\end{tikzcd}
\end{document}

I get this

How do I make it appear in the long horizontal segment at the bottom?

Answer 1

New answer: without changing the construction of the path.

By default, nodes have the option pos initialized implicitly. It is enough here to initialize to [pos=0.25].

\documentclass{article}
\usepackage{tikz-cd}

\begin{document}
\begin{tikzcd}[labels=description, column sep=3cm]
  A\arrow["a+b",rounded corners,
  to path={[pos=0.25]
    -| ([yshift=-1.5cm,xshift=-.7cm]\tikztostart.west)
    -| ([xshift=.7cm]\tikztotarget.east)\tikztonodes
    -- (\tikztotarget)} ]{r}
  & B \\ C &  D
\end{tikzcd}
\end{document}

Old answer: by modifying the construction of the path.

\documentclass[border=5mm]{standalone}
\usepackage{tikz-cd}

\begin{document}
\begin{tikzcd}[labels=description, column sep=3cm]
  A\arrow["a+b",rounded corners,
  to path={
    -| ([yshift=-1.5cm,xshift=-.7cm]\tikztostart.west)
    -- ([yshift=-1.5cm,xshift=.7cm]\tikztotarget.east)\tikztonodes
    |- (\tikztotarget)} ]{r}
  & B \\ C &  D
\end{tikzcd}
\end{document}