How did How I Met Your Father kept its surprise How I Met Your Mother cameos a secret?

TVLine

"We have a live feed that goes out to everyone who is working from home, so whenever we do bigger, secretive stuff like this, we cut the live feed so it’s very limited in terms of who is able to see," says co-creator Isaac Aptaker of the return of the two HIMYM fan-favorites on this week's episode. "And then we redact pages from scripts to limit distribution. We’ve learned from (co-showrunning) This Is Us to do what you can to keep things under wraps and try to surprise people."