Accessing sender's custom fields inside ISlashCommand's executor

I have a simple text-type custom field I would like to access from inside a slash command.
The field is defined in Rocket.Chat server’s settings:

	"Extension": {
		"type": "text",
		"required": false,
		"public": true,
        "private": false,
		"sendToIntegrations": true

Custom fields’ values should be accessible (I think) via the IUser.customFields? optional dictionary, but it seems it is not filled with anything.

public async executor(context: SlashCommandContext, read: IRead, modify: IModify, http: IHttp): Promise<void> {


rocketchat_1          | {
rocketchat_1          |   id: '3Gg7KDwQiGeY9Yp2w',
rocketchat_1          |   username: 'davide',
rocketchat_1          |   emails: [ { address: '', verified: false } ],
rocketchat_1          |   type: 'user',
rocketchat_1          |   isEnabled: true,
rocketchat_1          |   name: 'Davide',
rocketchat_1          |   roles: [ 'admin' ],
rocketchat_1          |   status: 'online',
rocketchat_1          |   statusConnection: 'online',
rocketchat_1          |   utcOffset: 1,
rocketchat_1          |   createdAt: 2020-12-07T11:05:51.516Z,
rocketchat_1          |   updatedAt: 2020-12-15T10:10:26.918Z,
rocketchat_1          |   lastLoginAt: 2020-12-15T09:51:42.335Z,
rocketchat_1          |   appId: undefined
rocketchat_1          | }
rocketchat_1          | undefined

where no attribute customFields is present.
Is this the right way to access custom fields?
I am using the rocketchat/ Docker image to deploy the slash command.