Backer Build 5 isn’t on the Xsolla launcher. It is on the Epic Games Store Launcher.
on March 27th you should have received an email from contact@snapshotgames.com titled “your key to backer build 4,” with a key to the Epic Games store. It could have ended up in your spam or trash folder, or your promotions folder if using gmail/hotmail. If the email address associated with your method of payment is different to the one you are searching, check in the other one. If you still cannot find it, send an email to contact@snapshotgames.com