I am really just following this answer.
\documentclass{article}
\DeclareFontFamily{U}{FdSymbolC}{}
\DeclareFontShape{U}{FdSymbolC}{m}{n}{<-> s * FdSymbolC-Book}{}
\DeclareSymbolFont{fdarrows}{U}{FdSymbolC}{m}{n}
\DeclareMathSymbol{\leftfootline}{\mathrel}{fdarrows}{"AC}
\DeclareMathSymbol{\rightfootline}{\mathrel}{fdarrows}{"AD}
\DeclareMathSymbol{\longleftfootline}{\mathrel}{fdarrows}{"C6}
\DeclareMathSymbol{\longrightfootline}{\mathrel}{fdarrows}{"C7}
\begin{document}
$A\leftfootline B$ $A\rightfootline B$ $A\longleftfootline B$ $A\longrightfootline B$
\end{document}

The question how to look up the code/coordinates for the symbol has been answered in this comment. Compile
\documentclass{article}
\usepackage{fdsymbol}
\usepackage{fonttable}
\begin{document}
\xfonttable{U}{FdSymbolC}{m}{n}
\end{document}
to get

To access the character, you can look up its hexadecimal "coordinates". The first one is on the right and the second one is the hexadecimal number of the entry. From this you see that \leftfootline is in the group with A and it is the 14th member of the group, so has "AC. This means we get it with
\DeclareMathSymbol{\leftfootline}{\mathrel}{fdarrows}{"AC}
The other entries are analogous.
MnSymbolandfdsymbolhas a\rightfootlinesymbol. You may just import this character, not load the full package, see https://tex.stackexchange.com/a/398874. – Feb 16 '20 at 08:24