To refer to e.g. A1 on Sheet1 while you are not in Sheet 1, use this:

=Sheet1!A1

If you have given the sheet another name, use that name:

=MySheet!A1

If the name has a space in it, put the name between ' :

='My Sheet Name'!A1

In the thread title you are asking about a dropdown but I don't see that in your file. Do you need help with that too?