The website vaxstandby.hidrb.com is legit. It is a part of vaxstandby.com, a website that lists leftover or extra doses of COVID-19 vaccine which must be used or thrown out, so nearby people looking to get vaccinated at a moment's notice can do so.
Vaxstandby's Website
To register with Vaxstandby, visitors are asked to go vaxstandby.hidrb.com.
If you want to verify the legitimacy of Vaxstandby, here some article published on popular news websites: