to set a value in matrix cell which is linked

In sales order if I enter form no of TAX TAB as "form c" , each cell of the TAX Code column of the matrix of contents tab should be set the value as "CST". I have tried to set the value, but it is showing "Form item not editable". I have tried to make the cell as editable but still the error message is coming and it is not setting the defined value. How can this be solved?

Thankx in advance