I've found the internet fairly unhelpful with this issue and thought I'd turn. I have the following arima model:

y = data$produced_diff
regs = data[,c(10,11,12)]
    
ar1ma1x = arima(y, xreg=regs, order=c(1,0,1))

Pretty simple. The data is daily and I'm trying to add a weekly seasonality. I'm currently doing it like this:

ar1ma1xS = arima(y, xreg=regs, order=c(1, 0, 1),
            seasonal = c(0, 0, 7))

I ... think this is right? It runs and gives slightly different output. But I find the documentation fairly opaque.