How to keep URL parameters on subsequent pages after a form is submitted?
If you're able, and willing, to review/rework the current implementation you have for your webform submissions and the related &quot;Thank you&quot; page, you should consider getting the Rules module involved, e.g. to have the Rules module react to Rules event &quot;After submitting a webform&quot;.
By doing so, you could then also use the Rules URL Argument module, which you can use to make the value of the URL argument available as new variable to Rules. Here is a quote about it (from its project page):

... provides two rules conditions based on URL arguments:

check if a URL argument is present.
compare the value of an URL argument.

It also provides an action that makes the value of an URL argument available as new variable to Rules.

The newly provided Rules actions and conditions can be found under the &quot;URL Argument&quot; conditions and actions groups in the rule configuration interface.
For an example of how to use it, refer to Comment # 3 in issue # 1686360 which is about &quot;Pass an amount to the url, ie; site/content/node-title?amount=10 or /node-title?=amount:10&quot;. Here is the relevant part of it:


add a &quot;Check if URL argument exist&quot; condition to your rule and set the &quot;ARGUMENT&quot; value to &quot;amount&quot;.
add &quot;Provide URL argument value&quot; action to the rule and set &quot;URL ARGUMENT NAME&quot; value to &quot;amount&quot; and in the &quot;Provided variables&quot; section &quot;ARGUMENT FROM URL - Variable name&quot; to e.g. &quot;amount_from_url&quot;.

You will now be able to use the amount value as a variable &quot;amount-from-url&quot; for other actions in this rule.