This is exceedingly annoying and totally unnecessary. In this day and age typically only bank sites won't remember a password. Who really cares about security on what they ate yesterday? And, since the password field is set to autocomplete off in the source code, it means password managers struggle to remember it.
There are ways around it, as others have mentioned, but if you don't want a browswer add-on and still want to solve it, you can create an HTML page on your own computer and mimic the functionality of the form submit on the fitday login page, except store your password within that local page. This is mildly technical, but I'm hesitant to provide the exact steps without a blessing from somebody at Fitday.