How do i add the placeholder in manifest.json parameters?

Hi Dojo,
 
You're using "text" type in your parameters. Text type is "a single-line input field". See the parameters list represents for Type. The following controls are available:

Type	Control
text	A single-line input field
checkbox	A single checkbox
url	A single-line input field, validated for correct URL syntax
number	A single-line input field, validated to include only numbers
multiline	A multi-line textarea. User input is limited to 64Kb
hidden	A parameter never shown to the user on the settings page
oauth	Required if your app uses OAuth. See oauth in the manifest reference. The parameter is never shown to the user

 
For more details, kindly check our Manifest reference.