Adding a referral link to an MQL5 signal page is not a built-in feature of the MQL5 signal service.
MQL5 signal pages typically display trading statistics and information about a signal provider's performance but do not provide a direct way to include referral links or external URLs.
However, they can include a referral link or external link in the description section of their MQL5 signal page. This can be done by editing the description via the signal settings page for the desired signal that is being copied by their followers.