There are several ways to accomplish this. To set a few registers or bits you can place
Set Bit or
Set Word objects on the common window (window #4). These objects have an option for
Set On when window opens:

Because the common window is always visible objects placed on this screen will be triggered once when the project starts.
Another option is to use a macro. The
SetData function can be used to write directly to PLC registers:

And the macro editor includes an option to
Execute one time when HMI starts:
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