How to use command to insert a space in insert mode?

Question

As title. Given that I'm already in insert mode and there is some insert mode function. Inside the function, I want to insert a space by command, how?

Answer 1

something like the following snippet works while in Insert Mode and typing <C-o>:call Testing()

function! Testing()
  echom 'testing...'
  exec 'normal! i '
  echom 'done.'
endfunction

(added it to my .vimrc for testing)