The correct definition is
- platform: mqtt
name: bedroom_cover
command_topic: "shellies/shellyswitch-xxxxxx/roller/0/command"
position_topic: "shellies/shellyswitch-xxxxxx/roller/0/pos"
set_position_topic: "shellies/shellyswitch-xxxxxx/roller/0/command/pos"
availability_topic: "shellies/shellyswitch-xxxxxx/online"
payload_available: "true"
payload_not_available: "false"
qos: 1
retain: false
payload_open: "open"
payload_close: "close"
payload_stop: "stop"
position_open: 100
position_closed: 0
optimistic: false
You can then click the card in the web interface and adjust the slider.