@shraddha priya I am sad that it doesn't come for you, because I remember when I filled the form, I got preference-1 field as active and it had only one option, which was same as the one which I selected during registration.
Could you try the step by step once again ?
 Use this link: http://www.iitg.ac.in/acad/Admission_July2019/mtech.html
 In the table there, click here do first time registration.
 Fill the details and note that you have selected correct academic program and discipline.
 Again, go to the above link mentioned and click in table, click here to login after registration.
 Now, fill in the personal details and preference-1 dropdown should have option for the same.