if a person wants to fill up the blank then how to write the code with respect to the asked question

Answer 1

Use :

<input type="text" name="fillup"/>

or

<textarea>

Answer 2

for a blank field you use "required" attribute
this attribute display a message if the field is blank in the form

Answer 3

either touch at the blank and type or if option are provided below tap the options after selecting a blank