how to link a parameter to a formula

I'm using Crystal Reports 2016. I have a report in which I created multiple parameters that successfully link to various fields. However, my current requirement is to link a parameter to a formula that I created but Crystal does not allow me to select this formula as the basis for this parameter. I tried creating a sub-report with a shared variable & linking this to my main report but Crystal does not allow me to select this shared variable either. Please advise. Thanks!