Dear aspirant

You haven't mentioned exam named or for which you are asking.If you have applied  any online application form for any exam then they might have send you application no. Or other details related to you on the mobile number which you have registered in the application form or on the email which you have provided during the filling of application form. You might check your phone and mail inbox you might find there. For safety anytime you fill any application form please take a soft copy and keep it safely.

Hope this information helps you

All the best

Assuming you are saying it as the application number for any exam. So if you have the confirmation page then you will get the number from that. If you don't have that then it will be a problem. Try to find our the confirmation page you get after the submission of the application form submission.

If you didn't have confirmation page then I will ask you to contact the organizers and get it sorted. If there is any  way out they will guide you out.

Have a great day!