(This assumes a WordPress.org site not one on Wordress.com hosting.)

Placing your username and password in the database of third party sites is not very good. If the account provided is the WordPress administrator account, then that means credentials for the most important account are potentially exposed. The password is going to be kept in the clear or in a form decryption is easy so it can be used to post to WordPress.

Better instead is to create a limited user with the Author role for this purpose. These accounts are so easy to create that I make one for every site I use to post to this blog. If any of these sites are hacked or the credentials otherwise given to others, then the potential damage is just the posts belonging to that user.

One stumbling block for this is WordPress.org installs want a unique email address for each account. A workaround I use is either generating email accounts via my hosting provider or the +anything for Gmail.

Also, it makes easy identifying the posts which came from the foreign source. My Goodreads posts are an example where that site is setup to post for an account I specially created for that purpose.