help with Instruction()

[Angus]

hi everybody!

I need help with Geometry.Instruction().
Reading from the list works fine, but doing

Geometry.Instruction(0):="plotfunc(x^2,x)" fails with error Invalid input.

f(x):=x^2/4;
Geometry.Instruction(0):="plotfunc(f(x),x)" fails with error Invalid Input.

edit:
I remembered Han's comments on strings as parameter in CAS commands in a program. So I tried
cmd:="plotfunc(x^2,x)
Geometry.Instruction(0):=cmd which also gives an error

How do I use that command to ADD a String to the list?