This option is automatically available when donors login to the donation page using their account information, under Optional Login. The link is listed at the top.

If a user does not have an account already set up then they will not receive this link. Or, if a user does have an account, but they do not log in under the same user account information (login ID) then NetDonors will not recognize them as the same donor, thus the giving history link will not be available.

In short, the link will only be available to donors who have set up an account and continuously log in using the same account information.