Deselecte the option "restrict cross site scripting" (in dutch "Het uitvoeren van scripts tussen frames beperken")
That usually saves a lot of troubles.
crmForm.all.name.DataValue = crmForm.all.IFRAME_CustomIFrame.Document.formname.name.value;
In this example you do need to change the name of the attribute to which you want to save the data to (crmForm.all.name -> crmForm.all.yourSchemaName). The next modification is to change the name of the IFrame. When setting up an IFrame, you do supply a name. Change the IFRAME_CustomIFrame to IFRAME_YourIFrameName. Then on your custom html or aspx page, you do need to access the variable. That you can do by specifying the name of the form followed by the name of the attribute.