Place holder Text

Good morning from Indonesi. im really a beginner on pdf

i have boss want to make a form. he want a form field that have default text $0, when we click that field we can put number that have format $ sign number.

im succeed to make placeholder text $0,  i use format script (Format>Custom> customformatscript>if (!event.value) event.value = "$0";)

But the problem is when we put text on that field, i cant make put $ sign by default. the format number is regular number without $ sign.

Could you help me

PS: im sorry my english is bad, hope you can understand

please see image to make it clearer

Thank you