Is your question about how you define or how you write? The first is related to the physical conditions at the boundary of your problem while the second id related to code syntaxe

Please guide me in both cases, I am going to write these boundary conditions in the input file but I do not know how?

For example, I know for uz=0, the displacement is zero and I found the keyword to write in the input file;

*Boundary

peripheral,3,3,0.0

Please help me with ∇c=0 , c∂u/∂t=0.