How can in I define a keystroke in Xemacs

Hello Xemacs users,

I want to define a keystroke with the command

M-x
append-to-file

This commands appends a marked region a file.

E.g., this should be the keystroke

M-x M-a

How can I achieve this??

Another question. Is there any command available like (let me say)
copy-to-file, which copies (instead of appending) a markes region to a
file?

Thank you for the help!!

Alexander