https://open.rocket.chat/channel/rocketchat-apps?msg=vuAxxx92rSZn4xEjP, https://github.com/RocketChat/Rocket.Chat.Apps-engine/issues/648

NabhagApp.ts

import {
    IAppAccessors,
    IConfigurationExtend,
    IEnvironmentRead,
    IHttp,
    ILogger,
    IModify,
    IPersistence,
    IRead,
} from '@rocket.chat/apps-engine/definition/accessors';
import { App } from '@rocket.chat/apps-engine/definition/App';
import { IAppInfo } from '@rocket.chat/apps-engine/definition/metadata';
import { IPreRoomCreateModify, IRoom, RoomType } from '@rocket.chat/apps-engine/definition/rooms';
import { ISlashCommand, SlashCommandContext } from '@rocket.chat/apps-engine/definition/slashcommands';
import { IUser } from '@rocket.chat/apps-engine/definition/users';

export class NabhagApp extends App {
    constructor(info: IAppInfo, logger: ILogger, accessors: IAppAccessors) {
        super(info, logger, accessors);
    }

    protected async extendConfiguration(configuration: IConfigurationExtend, environmentRead: IEnvironmentRead): Promise<void> {
        await configuration.slashCommands.provideSlashCommand(new (class _ implements ISlashCommand {
            command = "do"
            i18nDescription: string = ""
            i18nParamsExample: string = ""
            providesPreview: boolean = false

            async executor(context: SlashCommandContext, read: IRead, modify: IModify, http: IHttp, persis: IPersistence): Promise<void> {
                const usernames = [((await read.getUserReader().getAppUser()) as IUser).username, context.getSender().username]
                const room = await read.getRoomReader().getDirectByUsernames(usernames)
                await this.send(context.getRoom(), JSON.stringify(room), modify, read)
                if (room) {
                    return
                }

                const id = await modify.getCreator().finish(modify.getCreator().startRoom({
                    type: RoomType.DIRECT_MESSAGE,
                    creator: context.getSender(),
                } as IRoom).setMembersToBeAddedByUsernames(usernames))
                const direct = await read.getRoomReader().getById(id)
                await this.send(context.getRoom(), JSON.stringify(direct), modify, read)
            }

            async send(room: IRoom, message: string, modify: IModify, read: IRead) {
                await modify.getCreator().finish(modify.getCreator().startMessage({
                    sender: await read.getUserReader().getAppUser() as IUser,
                    room,
                    text: message,
                }))
            }
        }))
    }
}