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May we know more what you would like to accomplish here? I presume you needed to change the source URL in the <iframe> element?
If you want to embed an iframe element in your form, that is possible using our iFrame Embed widget. You can also embed it on the "Text" field, it allows HTML code to be injected there.
Here's a screenshot showing the "Text" field in our form builder.
I want to set a placeholder in the iframe element in a jotform, when it's launched, I can replace that placeholder with an url that's passed through request params.
As my colleague suggested, you can add iFrame Embed widget to add iFrame to your form. Unfortunately, the src of the iFrame cannot be changed dynamically based in URL parameter. However, if you have some knowledge using PHP to get URL parameters, you can use PHP in your own page and you can add your form on that page using its form source (How to get the Full Source Code of your Form). You can then manipulate the src of the iFrame in your form using PHP.