One way to do that is using a "Triggered subsystem" block:
https://www.mathworks.com/help/releases/R2022b/simulink/ug/triggered-subsystems.html
The triggered subsystem will become enabled when the control signal changes. By default, it is enabled when the control signal goes from a zero or negative value to a positive value.
The attached example records the time when a ramp signal crosses the value of 2. The ramp has a slope of 0.5, so the expected time of that event is 4 seconds.
As you can see, input of the "Triggered subsystem" is the simulation time using the "clock" block. The subsystem is simply wired so that its output is equal to its input. When the subsystem is triggered, it assigns the clock time to its output value and holds it.
