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If she's peeing when you put on her collar there could be two things going on. One is that she's giving you a submissive pee. She's showing you, in her own way, that you are better than her. She's frightened and is letting you know that you are the alpha.
The other cause could be that she just has to go so badly when you put on the collar that she just can't hold it. This happens a lot with puppies.
I suggest that you get a slip lead that you can slip over her head, pull to tighten and rush her outside to pee. After she does, THEN put her collar on and take the normal walk so she can finish.
This might take some time and patience but you can beat this.
So can she.
Having her spayed could help in a lot of ways. AND it's better for the dog. You will want to do it before her first heat if you can.
Let me know if you still have questions.