String.Format - moving from C++ to C#

String.Format - moving from C++ to C#

we are upgrading our code from C++ to C# and in alot of place we are
formating strings. for exmaple we have something like that:
OurString.Format("amount = %0.2Lf", (long double)amount);
How to convert the %0.2Lf into C# format ? i've tried the following code
but it's not the same
string formatString = String.Format("amount = {0}", (long double)amount));
thx