Have a solid research proposal and present it well. There is no other way to convince your assessors.
In my case, I had to do a public presentation with the assessors sitting in the audience. It was not an easy exercise but I took on the challenge head-on (really, there was no other option). And I prepared, prepared and prepared.
I asked my supervisor what the likely questions would be and how can I make my proposal better. I attended presentations by other students and studied the vibes.
Just be aware that not everything always goes to plan. I had chosen a concept that was highly contested in the literature so had several question on how I would handle it. (One of my assessors was an expert in that area!)
Most importantly, show a willingness to learn. If you cannot answer a question, be honest about it. Say something like "Thats an interesting angle and I will certainly take your comments on board". And mean it. Be honest and polite.
Confirmation of candidature is to show you are ready to do advanced research. Your assessors would most likely overlook a few flaws in your proposal (no research is perfect) but may not
be that generous if they realise you are not ready. Believe me, they are amazingly quick at the latter.