@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 ?
[1] Use this link: http://www.iitg.ac.in/acad/Admission_July2019/mtech.html
[2] In the table there, click here do first time registration.
[3] Fill the details and note that you have selected correct academic program and discipline.
[4] Again, go to the above link mentioned and click in table, click here to login after registration.
[5] Now, fill in the personal details and preference-1 dropdown should have option for the same.