Hi Slater,
You will need to store the existing ground profile into COGO so it can be used within the dialog - assuming you plan on the settings being relative to the existing surface profile.
Do you have the Profile Cell created and an existing surface profile drawn at this point?
Assuming you do, you can store that existing surface profile to COGO from within the Draw Profile dialog/tool.
The image above shows the tool you would use.
Select the ground profile row within the dialog (not shown) and then use the highlighted tool to store that selected profile to COGO.
You should be able to move forward from there.
HTHs