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.

Hope it will be useful :)