You are welcome to use whichever name you would like when you volunteer. Some volunteers like to tie their work to their fandom identity and others prefer to use their legal name, especially if they plan to use their volunteer service on their resume or CV. You are welcome to do either, or to choose a completely new name to use.
If you are accepted to volunteer as a Tag Wrangler but do not wish to tie your wrangling to your existing account, your chairs can provide you with an invitation to set up a separate account for wrangling purposes. If you want to use your existing account, it does not need to match your OTW name, but please be aware that your OTW name may be linked with your AO3 name in the course of your wrangling work.
Please note: A limited number of roles do require you to use your legal name, as they involve work with outside organizations. This will always be noted in the position description or the application.