Form Fields with text pre-filled

I have an InDesign form that will be distributed to University employees as a pdf.

The client asked if I could have a $ sign already filled into some of the fields, because these are fields where the employees is supposed to list expenses.

Giving them the $ will serve as a prompt.

So when they place a cursor into the field, it's going to be to the right of the $ and they start typing a number in.

How do I do that?