You would need to write a function which

1) identified the range to be multiplied

(probably select C1 and then extend selection end down)

2) stepped through the range multiplying each cell.value as it went

3) returned the final product

I do not have the time to do this just now. How about you try it and let us
know if you have problems.

The function would need to be in a Module not in the sheet code. Then you
just refer to it by entering its name (with the following parentheses) in a
cell.