you are asking this question on the wrong forum… thanks
@panda15sumitra when you scroll down in your mailgun dashboard, you will see four tabs that are Sending domains, Dedicated IP’s, Open tickets and Validations . Your sandbox domain is present in sending domains tab. In general, when we scroll down the dashboard by default the sending domain tab is on and we can directly get it but sometimes due to random click’s it can be possible that different tabs get open. Just scroll down you will get your sandbox domain.
Check this thread Mailgun signup page not opening
click on ‘sending’ on the left dashboard
under sending click on ‘domains’
in that sandbox domian will be listed