Mom is elderly and keeping up with test results, appointments, and messaging of healthcare providers isn't as easy as it once was. When I know she's awaiting a test result, I will ask her permission to check her MyChart, and I'll also work with her to send messages to her providers for care-related questions and concerns. Is this alright, according to all the rules?
If you send a message to a doctor or provider, they could ask you for the POA, so have it handy, but they also might not, especially if they have met you when you've taken your mother to appointments.
I would encourage you to have a HIPAA that never expires done by an attorney, that will allow you to do what you need to for the rest of her life.
If this is an online link for your mother's lab/hospital/testing, she should provide you with the sign ins that she uses. I use Kaiser. With my Kaiser password my daughter or my partner would be able to function to look at all charting and lab testing and etc on MyKP link.
If you have any questions, speak with your mother's medical team or the Hospital or Insurance program in charge of MyChart. They may want you to present them with you POA papers.