My room mate is thinking to claiming the rent as she is the primary on the lease for apartment. I am self employed how do I claim for my office space?

If you have a receipt showing what YOU paid for rent, then you would use that amount when entering your business-use-of-home expenses.  It will be prorated depending on the square footage used for business, and the total square footage for living area.

Thank you! Would the receipt come from her or property management? How would this affect her claim?

Your roommate should only claim the amount she pays.

As to what you need for a receipt...something from the landlord would be great, but at least something from your roommate stating you paid her for rent

Thank you this is extremely helpful