You can send both the Lookup ID and PIN together. However, if you are communicating over an insecure channel and you need to refer to the share, you can refer to it by the Lookup ID. For example, Alice writes in an email to her cat sitter Bob:

Hey Bob,

Thanks again for agreeing to feed my cat Luna. You are a lifesaver!

I am sending you the WiFi password in note XYZC, and the gate code in note QCTY.
You will need to use the Crosspass app to get them. Text me when you are at the gate.

Feel free to hang out at my place, Luna could use the company.

Hugs,
Alice

Then, once he arrives at the gate and texts her,

I am at the gate and I have Crosspass. What’s the PIN?

She texts him,

XYZC 1935
QCTY 0382