URL fields/18/en

From LimeSurvey Manual

To use passthrough fields, you must define the variable names via panel integration. If you don't assign a target question, the variables will not be saved into responses (read more about the LimeSurvey panel integration feature). To use such a parameter in the end URL, just insert the tag {PASSTHRU:<paramname>} where <paramname> is the name of your input parameter.