Set trendline intercept to cell value from spreadsheet

Hi,

I have an exponential decay data set, and want to force the y-intercept to take one of the values from the data set (the initial reading).

How can I set this up so that if new data is added, the intercept changes depending on the first value in the results table, please?

Can it be done easily or is it only possible via manual entry when 'formatting trendline'?

Many thanks in advance.
Conor