Web browsers can't use MQTT. There is a slightly hacky way you can achieve this however with some extra button clicks. In
Blue Iris you can add "On Call" actions to PTZ presets. So configure the delivery box cam as a PTZ (even if it isn't really PTZ-capable) and customize some of the PTZ presets to send your MQTT commands. To make it nicer in UI3, you can customize the preset button images by going to
Blue Iris 5/Cameras/{camera-short-name}
and putting in files named
preset_1.jpg
,
preset_2.jpg
, etc with the graphics you would like to see in UI3.