Thanks to your help, I managed to “debug” the code!
First of all, the URL in IFTTT was incorrect: the correct one was http://url:port/api/services/script/change_kitchen_channel?api_password=pass, and not http://url:port/api/services/script/turn_on?api_password=pass (even if, with scripts that don’t need variables, this URL works).
Now the code:
change_kitchen_channel:
alias: "Change kitchen channel"
sequence:
- service: media_player.play_media
data_template:
entity_id: media_player.kitchen
media_content_id: >
{% if (channel == "1") %}
http://iptv/link/1
{% elif (channel == "2") %}
http://iptv/link/1
{% endif %}
media_content_type: video