open about:debugging#/runtime/this-firefox

inspect “Firefox Multi-Account Containers”

paste the following function into Console

async function open_in_container(id, fqdn) {
	var uuid = Object(await browser.storage.local.get(`identitiesState@@_firefox-container-${id}`))[`identitiesState@@_firefox-container-${id}`]['macAddonUUID'] 
	return await browser.storage.local.set(JSON.parse(`{"siteContainerMap@@_${fqdn}": {"userContextId": "${id}","neverAsk": true,"identityMacAddonUUID": "${uuid}"}}`));
}

call the just defined function

await open_in_container(2, "example.com");

first parameter is the container id
second parameter is the fqdn of the site