If by "custom font" you mean a font that you are supplying as an asset, the following code should work:
Typeface plain = Typeface.createFromAsset(assetManager, pathToFont);
Typeface bold = Typeface.create(plain, Typeface.DEFAULT_BOLD)
Paint paint = new Paint();
paint.setTypeface(bold);
canvas.drawText("Sample text in bold",0,0,paint);
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