Power simulation for Difference-in-Difference

Question

Referring to the simulation approach suggested by Snow for ordinal logistic regression: https://stats.stackexchange.com/a/22410/231675

Here is a simple example with ordinal regression:

library(rms)

tmpfun <- function(n, beta0, beta1, beta2) {
    x <- runif(n, 0, 10)
    eta1 <- beta0 + beta1*x
    eta2 <- eta1 + beta2
    p1 <- exp(eta1)/(1+exp(eta1))
    p2 <- exp(eta2)/(1+exp(eta2))
    tmp <- runif(n)
    y <- (tmp < p1) + (tmp < p2)
    fit <- lrm(y~x)
    fit$stats[5]
}

out <- replicate(1000, tmpfun(100, -1/2, 1/4, 1/4))
mean( out < 0.05 )

I am looking to do the same thing for Difference-in-Difference Analysis. I am not sure how to adjust the parameters to fit a Difference-in-Difference. Does anybody have any suggestions?