How can an input of form1 can be displayed on the textbox of form 2?

Not automatically, no - but it's not difficult to do yourself.
The "relationship" between the two forms is important here: assuming the Form1 creates the instance of Form2 then there are basically two ways:
1) Create a Form2 constructor that takes a string parameter and pass the textbox content when you create the instance - this sets it once, and doesn't change it again, and can be used with Show or ShowDialog.
2) Create a property in Form2 and have Form1 set it from the instance it creates. This allows you to change the value in Form2 when it changes in Form1, and only really works with Show, not with ShowDialog.
If that isn't the case, then have a look at these:
Transferring information between two forms, Part 1: Parent to Child[^]
Transferring information between two forms, Part 2: Child to Parent[^]
Transferring information between two forms, Part 3: Child to Child[^]
They all contain example code which shows it in practice.