Generating Unique Username and Linking the sheets


I have a sheet which collects participants information, where the individuals who are interested in the enrolling to research study will get registered by submitting the form. And we need a column which captures a username of their interest (or at least by giving them a choice to enter their username by specifying a suffix or prefix in the help text under the field.) And within the sheet we will have an internal automated ID for every submission.

Now, I have another sheet which captures the information from participants who registered after the study period begins, and I want to link both the sheets based on username (because this will be the only common column in both sheets). Even though the participants enter the wrong username they will be able to complete and submit the form. So how do I connect both the sheets in this case? Are there any other possible workarounds? Any help is appreciated.