Sitting on eczema can be painful. Also the skin can get very tight and rough and when you sit i may pull and break the skin. I would get a referral from the Gp to see a Dermatologist for her eczema.
In the meantime I would take the toilet training low key. Set a potty in different parts of the house so she can go on the potty when she wants and needs to, praise her for when she does but no pressure or negative response when she doesn’t. Watch some video’s on Youtube kids about toileting. When I started toileting my youngest I did this daily. I also gave her a LaLaLoopsy doll that can pee and poo and comes with a potty

I’m not sure I can picture how what she does on the potty is affecting what’s on the back of her legs. Sitting on the potty or toilet……there is no way the wee should be coming into contact with the back of her legs. I hope it’s all sorted now

Hope ur having some luck toilet training.All I can think of is show it’s ok to use the toilet by u letting her see it’s ok by weeing ,you in toilet .

If you really think it’s the eczema I would teat it first,pure coconut oil you use for cooking is great if you not already using something , do you think she is ready , sometimes we are ready but the child isn’t

As grose as it sounds leave the door open and have her watch you go to the toilet. If you have other older children that go make them sit when they go and show her. The more she sees that you guys do it all the time the more comfortable she will be. I leave the door open for my son since he started walking at 13 months and he is now 23 months he has been telling me “toilet” for months. Even though he doesn’t always make it…it’s a start that I’m happy with. I also sit on the floor near him and have a conversation with him. When he goes wee or poo I make a big deal about it and praise him. He gets so excited that he calls everyone else in to see how good he has done

Do you have friends with children about the same age who are toilet trained? My daughter would hold and hold until it was too late. It wasn’t until we were at my sister in laws house and my 3 year old niece showed my daughter how to use it and actually took her to the toilet.
I also used every distraction I could think of. I would read stories or make up stories while she was on the toilet. We would sing. I put the play school app on my iPad for her. Left her favourite toy in the bathroom so if she wanted to play with it she could only do it on the toilet.
Good luck with it all. I think medals should be handed out for toilet training. Easily the hardest and most trying experience of my life.