Security is important when it comes to your website. Don’t share your username and password to anyone. The best way to give access to your WordPress website is to create a new user for them.
Your designer will receive an email with their password and instructions on how to log in to your WordPress website. They can then update and choose their own password.
If you are planning on having ongoing support from your website designer then you can leave their user account open for them.
Otherwise, once they have finished their work with you, delete the account so they no longer have access.