Mom locks the door and refuses help in bathroom when trying to get urine sample.

Mom is living in AL? (Assisted Living?)

Get a hat. Quest was the first time I was given one for Mom. It fits in the hole of the toilet seat, after they go you pour the urine into the container given. I so wish I had one at home.

Take the lock off the door.

depends on funds but maybe a Lightweight Foldable Bedside Commode  - about £100. ( a toilet with a bed pan built into it) - she would just sit on it like an ordinary toilet then someone with gloves can transfer it to a container? Just a thought if funds allow.
I'm not sure if a bed pan can fit over a toilet to sit down on? .... worth exploring.
(if funds not available - stand over a large bucket on the floor on a wipe down cloth or towel?

You can try a toilet "hat"

I wonder if it is time for palliative care? Trying to get a clean catch from an elder whose brain is broken … 🤦‍♀️ This may be another thing to add to my advance directive of procedures and treatments not to do if I become cognitively disabled.

I’m so sorry. This is part of the tragedy of the disease.

Remove the locks from all the inside doors including the bathroom! The last thing you want is an Alzheimer's patient locking themselves inside a bathroom where they can cause chaos and endanger themselves by eating soap or causing a flood in 2 minutes! If mom continues to be uncooperative at home, she'll have to move into managed care of some kind. Dementia often becomes unmanageable at home, which is why Memory Care Assisted Living homes are popping up on every street corner across the country.

Good luck.

Im's so sorry.

Do you think you may be coming to a time when in home care is no longer an option for the family?

Do understand that a "clean catch" as they call them is almost impossible at a certain age.
It requires A) a good cleanoff of the area with a sterile wipe B) the ability to start a stream of urine, then stop it and manuever a sterile container into position. Few elders can manage any of that. I am afraid that if you are talking a clean specimen the best you can do is have mom clean off with a washclothe and use a new "toilet hat" to collect urine and get it in as a clean catch OR have mom go in for a cathed specimen. Problem there is that a cath introduced often CAUSES infection, so it's a catch-22.

I sure wish you the best. I will never forget my now deceased friend's description of getting a "clean catch" from her Mom and all the while singing to herself "Happy Birthday to me".
What you all go through is more than I can imagine.