Completing the field to a certain number of characters

Dear members,
I hope someone will be able to help me. I have only recently started working with Adobe Acrobat Pro.
I have created a fillable pdf file.
The file contains checkboxes as well as text fields.

I need to fill one field automatically to a certain number of fields.
I found the padStart method but somehow it doesn't work for me.
Does padStart work in Adobe Pro?