*Then they need to insert a security token, sent to their email address to verify
No. This is not just non-idiomatic, it's difficult if not impossible to understand what you mean without an explanation.
Then they need to insert a security token, sent to their email address to verify this very email address.
This is a correct use of very as an adjective. Very in the sense of “the one just mentioned, emphatically” is an adjective, it needs a noun after it. But the sentence sounds clumsy.
Then they need to insert a security token, sent to their email address to verify the address.
That sounds right, linguistically speaking. But from a technical point of view it doesn't make sense to me: a security token is a physical object (at least in this context, but the sentence claims that the security token is sent to an email address. If the token is some kind of verification code (a nonce, in technical terms), and you don't want to use the technical term “nonce”, then you can call if a (one-time) verification code, and that code would be entered or typed rather than inserted.
Then they need to type a verification code which was sent to their email address to verify the address.
If that's not what you mean then this sentence needs more work.