I have an existing login page. Can I integrate Azure B2C into it instead of using the login screens provided by and hosted by Azure AD B2C?
A similar question was asked 4 years ago but the answer is most likely out of date: Can I use my own UI to authenticate users with Azure AD B2C?
You could have a look at the embedded sign up / sign-in option.
"For a simpler sign-up or sign-in experience, you can avoid redirecting users to a separate sign-up or sign-in page, or generating a pop-up window. By using the inline frame HTML element, you can embed the Azure AD B2C sign-in user interface directly into your web application."
If you are really determined to use the form of your own which has nothing to do with AAD B2C then the only solution you're left with is a Resource Owner Password Credentials grant where you explicitely send username/email and password over to AAD B2C and get a token in return.
Here's docs ref: https://docs.microsoft.com/en-us/azure/active-directory-b2c/add-ropc-policy?tabs=app-reg-ga&pivots=b2c-user-flow
Be aware though that this flow is not suitable for modern applications for security reasons and in principle should rather be used only for legacy solutions.
