Passing and storing Prolific Panel ID

I am using Prolific as a provider of respondents for the first time (rather than usual panel).

I want to pass in and store ID.
The help page:
https://researcher-help.prolific.co/hc/en-gb/articles/360009220993
indicates that the member's ID is:
PROLIFIC_PID

If I try to add PROLIFIC_PID
as a password field or pass-in field I get an error (I presume the underscore is the problem).

Am I making a mistake? Is there a way to pass in and store PROLIFIC_PID?

thanks
Dan