Form block customisation

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Hi Guys,

I am using the in-built Form block in one of my web sites. What I need to do is pass a GET variable to my form (easy enough) and ensure that the value of this variable is included in the e-mail that is sent to the form recipient (struggling with this).

So e.g. /bookingform?code=ACBCDE

When the form is successfully submitted and the email is sent, I would like to insert "Code: ABCDE" in the resulting email.

Could someone point me in the right direction?

Many thanks