Add default value to long description field.

It is same as assigning default values to other fields.

Go to Application Designer --> Select your application --> go to require section --> drag and drop default value control from control palette --> select control properties for default value :

Attribute : Click on lookup and select attribute as DESCRIPTION_LONGDESCRIPTION for your object.

Value : <Add your default text>

Default Type : Insert <this will work while inserting new record in your application>

Save the application designer and try to insert new record in your application. Long description field will display your default text value entered in VALUE field.