To add the details block under an input, use the note attribute.
formData = [{ type: "input", name: "prj_name", label: "Project Name", note: { text: "Enter your project name here. This field is required.", width:300 } }];
There are 2 methods you can use to set add/remove the note block dynamically:
myForm.setNote('prj_name', {text:"I'm the note block"}); ... myForm.clearNote('prj_name');