This story is just weird enough to go down in history as one of those great actor as character origin stories. Like Ryan Reynolds picking up an issue of Deadpool only to see the character describe himself as looking like "cross between a Shar-Pei and Ryan Reynolds," after being given the comics by an executive on Blade: Trinity, Chadwick Boseman has his own story of being given comics of his comic book role.
On Live with Kelly and Michael, Boseman told this story about the "sign" he got:
I got in a conversation with the security on set for this movie called Gods of Egypt in Australia, and we didn't really even talk about Black Panther at all, but he had them put the first issue of Black Panther -- his original issue from when he was a kid -- in my trailer and wrote a note that said, 'you're going to get this role'.
He better get that thing framed. Here's the segment if you want to watch it:
Image: Captain America: Civil War, Marvel/Disney