@silicuda yes if anyone press get updates button, they will subscribe your bot.you can send them any update or promotional messages after 24 hours. 

My question is,    how else can I get them to Subscribe so I can send promotional messages?  What action can I use in manychat?

In other words which action === same as someone pressing the Get Updates butoon?

@silicuda you can't, the only way to trigger the "Get Updates" button is using the Messenger Lists and getting an explicit click from the user. There's no Action with the same functionality.

This is a limitation from Meta unfortunately.