message-indent-citation

message-indent-citation is a compiled Lisp function in `message.el'.

(message-indent-citation &optional START END YANK-ONLY)

Modify text just inserted from a message to be cited.
The inserted text should be the region.
When this function returns, the region is again around the modified text.

Normally, indent each nonblank line `message-indentation-spaces' spaces.
However, if `message-yank-prefix' is non-nil, insert that prefix on each line.