Go to your plant list. I use a desktop so what I post here may be incorrect for you if you are using a different device. At the middle of your plant list page is a green bar/button that highlights yellow when you hover over it. It says "Add a Plant to Your List". Click on that. It takes you to a page that looks like this
Then type in whatever you want to name your plant. And click "Proceed to the Next Step".
It then brings you to a page where you can select if you want/have the plant, add notes or a photo ect. Then click "Save Changes" at the bottom.
You can add any photo or name you want, as it is not going to go into the database. This only shows up on YOUR plant list for your use. For example I have some daylilies I don't know the name of and I just made up my own names, no one will know.
What I do now, is if I find a plant I want to add to my list and it is not in the database-instead of going to the hassle of adding it (a pain I don't deal with) I just add it to my list this way. Today I added "Yewtopia Plum Yew" since I could not find it in the database. It is a plant currently being sold by "Southern Living Plant Collection", I did find some plants of that brand at Lowes last summer, so maybe they will have one this year.