Send variable in JSON possible?

I am currently trying to implement a use of the litter boxes via WLED.
I would like to create a display and read the stop value of a segment from an item for this. Is this possible at all?

Via Telegram I can read out the variable, it looks like this:

telegramAction.sendTelegram("<b>Katzenklo Rechts</b>\n\nDas Katzenklo wurde heute bereits <b><u>%s</u></b> mal benutzt", AZ_Katzenklo_Benutzungen_Rechts.state.toString)

My MQTT command looks like this:

AZ_Katzenklo_WLED.sendCommand('{"on":true,"bri":100,"transition":7,"mainseg":0,"seg":[{"id":0,"start":0,"**stop":3**,"grp":1,"spc":0,"on":true,"bri":255,"col":[[255,0,0],[255,92,163],[84,161,255]],"fx":0,"sx":185,"ix":112,"pal":9,"sel":true,"rev":false,"mi":false},{"id":1,"start":7,"stop":9,"grp":1,"spc":0,"on":true,"bri":255,"col":[[255,0,0],[255,92,163],[84,161,255]],"fx":0,"sx":185,"ix":112,"pal":9,"sel":true,"rev":false,"mi":false},{"stop":0},{"stop":0},{"stop":0},{"stop":0},{"stop":0},{"stop":0},{"stop":0},{"stop":0},{"stop":0}]}')
            

Now I would like to read the stop segment from my item and use it. Is there a way to do this?

Sure, use JSONPATH Transformation directly from the rule: (by the way, your json is not correct)
Now, there are many stops in the JSON Object, which one do you want?

{
	"on": true,
	"bri": 100,
	"transition": 7,
	"mainseg": 0,
	"seg": [
		{
			"id": 0,
			"start": 0,
			"stop": 3,
			"grp": 1,
			"spc": 0,
			"on": true,
			"bri": 255,
			"col": [
				[
					255,
					0,
					0
				],
				[
					255,
					92,
					163
				],
				[
					84,
					161,
					255
				]
			],
			"fx": 0,
			"sx": 185,
			"ix": 112,
			"pal": 9,
			"sel": true,
			"rev": false,
			"mi": false
		},
		{
			"id": 1,
			"start": 7,
			"stop": 9,
			"grp": 1,
			"spc": 0,
			"on": true,
			"bri": 255,
			"col": [
				[
					255,
					0,
					0
				],
				[
					255,
					92,
					163
				],
				[
					84,
					161,
					255
				]
			],
			"fx": 0,
			"sx": 185,
			"ix": 112,
			"pal": 9,
			"sel": true,
			"rev": false,
			"mi": false
		},
		{
			"stop": 0
		},
		{
			"stop": 0
		},
		{
			"stop": 0
		},
		{
			"stop": 0
		},
		{
			"stop": 0
		},
		{
			"stop": 0
		},
		{
			"stop": 0
		},
		{
			"stop": 0
		},
		{
			"stop": 0
		}
	]
}