Hey,
I have military symbols as MultilayerPointSymbol.
Now i want to have a solid red border around this symbol.
I played around with composite and SimpleLineSymbols, but it don't worked very well.
What is the best possibility to show a solid line around my symbol?
To do that, you'd have to create a custom symbol dictionary using ArcGIS Pro. However I believe if you do this, the symbol would no longer be according the military symbol specification.
Are you just trying to highlight features with a red outline? (You could just set the highlight color to red, and select it)