OPTION 1
Step 1: Click on the 'Settings' icon for the desired file.
Step 2: Scroll down to the 'Default File Message' section, enter a message and click 'Update' to save.
When the file is shared, the 'Message' section of the Share Panel will autofill with the 'Default File Message.'
Note: The default message can easily be updated directly in the Share Panel by clicking 'Update Default Message.'
OPTION 2
Step 1: Click on the 'Share' button for the desired file.
Step 2: Enter a recipient's contact information and desired security permissions.
Step 3: Enter the desired text in the 'Message' section and click 'Save Default File Message.'