The purpose of this study is to evaluate the long-term safety and effectiveness of a medicine called NNC6019-0001 in people who have heart failure due to transthyretin (TTR) amyloidosis (build-up of abnormal protein in heart cells). People with heart disease due to TTR protein build-up have changes in a specific protein circulating in the blood, called transthyretin. This changed protein can be harmful, as it can make the risk of heart failure or other complications higher. Previous studies show that NNC6019-0001 may be helpful in treating people with heart failure due to TTR amyloidosis. Procedures include electrocardiograms, echocardiograms, and magnetic resonance imaging, blood samples, walking tests, and questionnaires about quality of life and health status. Researchers also hope to better understand how NNC6019-0001 works in the body, as well as how to improve the treatment of people with a heart disease due to TTR protein build up or related diseases. This study is an extension of a prior study investigating NNC6019-0001; only participants who have completed that study are eligible for this one. All participants in this study will receive NNC6019-0001, regardless of whether they received NNC6019-0001 or placebo (inactive substance) in the first study.