You cannot "pass credentials" to WOPI since credentials for Office 365 applications come from a separate system. You have to get credentials before you are allowed to access anything in Office 365. Basically Azure AD is the service & the login is done via login.microsoft.com, the login provides a token to your browser that is exchanged with the server on every request. To reuse an existing credential, you have to be using an application that "knows" you have already logged in. Typically, Microsoft use a helper application that picks up the login from IE if that's how you logged in and makes it available to other applications such as Office. If you are using Firefox to log in, IE & Office may not know that you have done so (though there is a plugin for FF that gets installed if you let it which does the same thing).