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LimeSurvey version: LimeSurvey Community Edition Version 6.10.5+250217 
Own server or LimeSurvey Cloud: Own Server
Survey theme/template: irrelevant (fruity_twentythree)
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I have an anonymous survey which I want to send out to different groups of people using a prefilled code in the URL, in the form of the following:

https://server/limesurvey/index.php/485329?CODE=group1

Where 'CODE' is the question code of one of the questions.  
The question is 'hidden'.

However, what can I do to stop people from deleting that from the URL and just enter the survey using:
https://server/limesurvey/index.php/485329

I tried to make the hidden mandatory and do input validation.  However, it seems that they are ignored.

Is there anyway I can do input validation for a hidden question?
 

Use an equation to check, like
{if(is_,empty(CODE),0,1)}
Now you can set a quota on the equation and screen out, if the value is 0.

But this doesn't recognise a changed parameter.
My first ideas:
1. Check if the parameter is in a list of all possible.
This list you can create as default value in a "long text" question (hidden). Check with an equation like {if(strpos(list,CODE)>0,1,0)}

2. Use parameters that you calculate a value by some arithmetic operations. If the "magic" value is not met, screen out.

Joffm