Errata....

I wrote:
Do you mean that total sales increases 5% per year, and you want to
apply the monthly rate equally to each month (surprise!) so that you
get a 5% annual increase?
In that case, the monthly rate can be computed as (in A1, for example):
=rate(12, 0, -1, 1+5%)

On second thought, that does not result in a 5% annual increase. It is
nonsense in this context. Sorry.

Or do you mean that you want a year-over-year increase of 5% applied to
each month?

That results in a total 5% annual increase.

The total forecast for the year is simply SUM(C1:C12).

..... Which is all you wanted.

The annual
percentage increase of total sales is (formatted as Percentage):
=sum(C1:C12) / sum(B1:B12) - 1

This is necessary only if you want to account for any anomaly caused by
rounding.