How to refer to variable of dataset in reactivity

I'm making a Shiny app. I use reactivity programming for better fuctionality. But I don't know how to refer to dataset that is reactive object. Here is the example:

buffer_bank <- reactive({mydata <-
        merge(buffer_data(), rwa())%>%
        mutate(buff = case_when(REGN %in% (szko$REGN) ~ (buffer + buffer_szko), TRUE ~ buffer))%>%
        select(REGN, buff)
    mydata})

Where buffer_data() and rwa() are reactive objects that is why I passed () after datasets in both cases. But how to refer to REGN variable in szko dataset, which is reactive object too.

How can I handle this problem?

Many thanks.