You have already answered your own question. This line...
MembershipUser newUser = Membership.CreateUser(UsernameTextbox.Text, PasswordTextbox.Text);
will effectively create the new user in the membership database. Then, notice the object returned from the method call is the newly-created user which exposes a property named ProviderUserKey
. This property holds the underlying user id which you will need to cast to whatever data type the membership provider uses usually a GUID
or an Integer
Guid userId = new Guid(newUser.ProviderUserKey.ToString());
or
int userId = (int)newUser.ProviderUserKey;
Then, you can add the required info to your business-specific tables (Client, Doctor). Another approach will be extending the membership system, but this will require a bit more effort on your side