I hope you have already resolved your issue and I am including it here for other users.

you can use Chrome's Developer tool to check the Post command and this example below is what I'm currently using to control Recalbox through Homeassistant

These commands are for Recalbox 9.2 New Web ui

###############################################
rest_command:
emulationstation_reiniciar:
url: http://172.16.8.6/api/system/frontend/restart
method: POST
headers:
accept: "application/json, text/html"
user-agent: 'Mozilla/5.0 {{ useragent }}'
payload: '{}'
content_type: 'application/json; cha**et=utf-8'
verify_ssl: false

emulationstation_parar:
url: http://172.16.8.6/api/system/frontend/stop
method: POST
headers:
accept: "application/json, text/html"
user-agent: 'Mozilla/5.0 {{ useragent }}'
payload: '{}'
content_type: 'application/json; cha**et=utf-8'
verify_ssl: false

emulationstation_iniciar:
url: http://172.16.8.6/api/system/frontend/start
method: POST
headers:
accept: "application/json, text/html"
user-agent: 'Mozilla/5.0 {{ useragent }}'
payload: '{}'
content_type: 'application/json; cha**et=utf-8'
verify_ssl: false

recalbox_reiniciar:
url: http://172.16.8.6/api/system/reboot
method: POST
headers:
accept: "application/json, text/html"
user-agent: 'Mozilla/5.0 {{ useragent }}'
payload: '{}'
content_type: 'application/json; cha**et=utf-8'
verify_ssl: false

@lorie3000 I had this happen to me It turned out it was the SSD to usb adapter I was using, the raspberry pi 4 didn't like it. I switched adapters and it worked perfectly.

Might not be your issue, but its a direction to try.

@srounsroun how do I get this to type on the line that needs to be changed? I put the recall boxes password in and I got a number sign a pound sign. Can you please give me some information as soon as possible on what to do here I am running the pie trying to load Pulstar 9.1 and I am able to get the password and username in and that’s all the further I’m getting

@djpcpower Did you ever find the cause of this periodic screen blanking? I would be very happy to hear what was causing it, as I'm sure it's the same cause for it here, and I'd really like to stop it! 🙂 Thank you for any advice!!! John