Tell them you will get a signature specialist to analyze the signature compared to yours and if is yours you will apologize and leave the company immediately.
If its forged you will sue them for forgery, defamation, stolen, identity and emotional abuse etc with a nice hefty paycheck of say a million dollars and mention that you will go public disclosing the company misconduct on social media tell the clients that the company forges signatures etc. Then ask them how would they like to proceed?