We did innovis online...do not have to join, just freeze...then they sent confirm letter
I froze my credit 8-10 years ago. I recall one agency was easy to do. I did it online. Another I think I had to send an email. However, one required a letter through the US mail. Then they replied with a letter and the code number I should keep in a safe place to unfreeze it easier in the future.
Since then my wife froze hers. Then my son had someone in MA file for benefits in his name. He was living in a low life town and someone probably stole his mail. He received a letter from the Massachusetts Unemployment office asking him to complete his application for benefits. He spent a day cleaning up that mess. Had to go in person to say I did not file for benefits. Scum bag scammers.