Do you want to pass a Parameter to PCF control within your canvas app?
Based on the needs that you mentioned, I think the OnStartproperty of App and a available could achieve your needs. You could consider save your parameter value into a variable within the OnStart property of App, then you could reference the variable value within your PCF control.
Set the OnStartproperty of App to following:
Set(ParameterValue, "xxxxx Specific Value xxxx")
The OnStart property of App would be fired when you run your app (before the DOM is fully rendered within your app screen). You could reference the variable within your PCF control directly, then your PCF control would be rendered before the DOM is fully rendered in your canvas app screen.
If you want to pass a parameter value to your canvas app through the App URL Link, you could append a parameter value to the App URL link, the sample App URL Link along with parameter as below: