Termination of parental rights must be on a finding of unfitness of by agreement. If you agree to the adoption of your child by the mother's husband, offer to send a properly executed consent. You would have to appear before a judge, bring a picture ID, and execute the Final and Irrevocable Consent to Adoption. Yes - contact her lawyer with this offer. You would not be able to change the wording if you do not voluntarily relinquish.
Before you make this decision, I highly suggest that you talk to an attorney. You don't have to hire one, just consult. You can talk directly to your ex-wife's attorney regarding changes you wish to be made.
Nothing in this post creates an attorney-client relationship. This post is not meant to serve as legal advice.