You will first need to add a Name/identifier of a respondent question, such as a single-row text, and then Logic>>Skip logic. Here, you will need to branch this question to the next question, followed by assigning a variable like custom 1 in the 1st survey.
Now go ahead and publish the survey, and then you will need to edit the Live Survey link by adding the Custom 1 at the end of the URL like (custom1=${custom1}). In the analysis of the second survey, you will be able to see the Custom 1 column appended with the respondent"s identifier
Help files on the above:-
1. https://www.questionpro.com/help/custom-variables.html
2. https://www.questionpro.com/help/comment-box-question-type.html