Typically the petition for adoption is accompanied by a petition for termination of parental rights. You are entitled to service of both, and the least costly response would be to do nothing. After 20 says default can be taken. A more proactive, albeit costlier, approach is to retain counsel to prepare a voluntary relinquishment which must be signed in front of a Magistrate. That document can then be filed with the petition for adoption and you would not need to be served with process.
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This answer is offered as a public service for general information only and may not be relied upon as legal advice.