Plastic hot water connection. Which compound to use??

I doubt the thread seals the connection.
There will be an olive of some description around the pipe to seal the fitting.

You could be in a world of pain!

Find the type of pipe or fitting and replace with new.

I personally would stuff around with that old fitting

To ensure a durable, leak-free connection for your concealed shower unit’s hot water inlet, start by using PTFE tape or liquid PTFE thread sealant. Wrap 3-5 layers of PTFE tape around the male threads in a clockwise direction, or apply a liquid PTFE sealant suitable for hot water. Silicone-based sealants can also be applied around the threads for extra protection, ensuring the sealant is rated for high temperatures and plumbing use.

If leaks continue despite using tape or sealant, consider switching to a brass compression fitting for a more secure connection. Avoid overtightening to prevent damaging plastic threads or warping the fitting. Before you begin repairs, turn off the water supply and clean the threads thoroughly. After making the repair, check for leaks over a day before retiling to ensure a reliable seal.