What do you have already? Relatively simple case:
1. Establish the imperative - if we don't have mandatory donations, there will simply never be enough organs (see: statistics).
2. Explain why it is morally justified - how somebody's right to life is much more valuable than your right to what happens to your body after death (because the former informs the quality of their life to a much larger extent - it influences how much happiness they can experience, the choices they can make, etc. The latter only becomes a minor annoyance for people - you are rarely so invested in your body post-death that it actively detriments how much you can enjoy life). Explain how even if there was a case where your body mattered after death, social attitudes are often artificial, and so over time the existence of this policy would normalise the idea that your body-after-death isn't something you should actually care about.
3. Explain the broader consequences of this - how making organ donation mandatory will raise awareness for the situation concerning organ transplants in general, but also the lack of resources of the medical system and how it needs so much more assistance from everyone. This might incentivise action with other areas (eg. more blood donations too, more lobbying for medical investment, etc.)