My employer (OH based) doesn't withhold state income tax for Wisconsin and doesn't have an employers state ID (box 15). How do I fill out the employers state ID number?

You don't need to enter the employer's state ID number when you enter your W-2 form if there is no entry in box 15, meaning no state is reported on the form. Since you are filing a resident tax return in Wisconsin, your federal income will be transferred to the Wisconsin state return, so you don't need to designate it as Wisconsin income.