When you want to use Mastodon API, you need access_token for your application. Using this site, you can get new access_token of your Mastodon site.
You can see all applications you've authorized in your account's Authorized apps settings.
Maintained by @prplecake@compostintraining.club
Source code: https://github.com/prplecake/pleroma-access-token
This page uses your client_id, client_secret and access_token, but the site is pure JavaScript.
If you can't trust this site, you can copy this page, or clone the repository, and host it locally any number of ways. This page uses Vanilla JS, so you can easily confirm what it does.