how to use telescopes `previewers` functions?

Question

I would really love to make use of this function

previewers.git_commit_diff_to_head() *telescope.previewers.git_commit_diff_to_head()*
    A previewer that shows a diff of a commit to head.
    The run command is `git --no-pager diff --cached $SHA -- $CURRENT_FILE`

    The current file part is optional. So is only uses it with bcommits.

but everything i try seems to error out. I've tried putting it in init.lua file as require('telescope.previewers').git_commit_diff_to_head() as a mapping inside telescope, as well as a key mapping outside of the init, but neither has worked