Running an extended command with default input?

[egarrulo]

How can you run an extended command with a default input?  For example: 
`eval-expression` with `(+ 1 1)` as default input (in other words: what 
you would accomplish by typing `M-: (+ 1 1)` without RET).  This code 
works, but it seems to add an implicit RET:

(execute-kbd-macro (vconcat (kbd (substitute-command-keys 
"\\[eval-expression] "))
                            "(+ 1 1)"))

Thanks.