Someone else posted a video that covered this. Some dishwashers (not mine) have a pre-wash spot for putting the soap. In the video, the guy had his spot actually on top of the soap door, so it would be measured but falls straight in with the dishes when you close the door for the pre-wash. He said in the video to just add a tablespoon of dishwashing powder on the door or in the bottom if yours doesn’t have a prewash spot.

I have learnt a lot from that video and all the comments here!