How to match a web form with a mail template at the code level in Bitrix?

Question

1C-Bitrix

When processing web forms, their fields are of the form form_text_ID (where ID is a numeric identifier). At the same time, a mail template is attached to the form, in which, for example, there is a #CLIENT_NAME# field. How can I match the web form field form_text_3 with the template field #CLIENT_NAME# at the code level?
Actually, this is necessary to send a letter using the CEvent::SendImmediate method