This is a thermostatic valve without water function (the picture below is a thermostatic valve with automatic water supply).
There are two solutions.
First, use a three-way valve, with two ports connected to the cold water pipe pointing up from the thermostatic valve, and one port connected to the tap water . And install a valve on the tap water connection pipe. This valve can be opened to supply water. Close it when the water is full and open it when taking a shower to drain the water.
Secondly, adjust the water volume of the thermostatic valve to the maximum, set the temperature to the minimum and turn off the shower head, then you can achieve the purpose of filling the water with this thermostatic valve without a water mixer switch.
Simple and easy to understand explanation! Mixer tap: you can adjust the water temperature yourself. For example, if you feel that the water is too hot during washing, you can adjust it yourself! Thermostatic valve: it regulates the temperature when in use, then when in use, there is a temperature adjustment inside the solar to keep the temperature within the set temperature range!