Can anyone help me in inventory variable in Ansible?

I have a server where root login via ssh is not permitted and I used to login on server with a normal user (MYUSER) and then `su -` to become root. 
I have ssh key-based authentication with user (MYUSER). 
Now, I want to execute something like below. 

```
ansible --private-key /PATH/TO/PRIVATE/KEY.KEY -i INVENTORY -m shell -a "whoami" all

```

But I am getting output as MYUSER. 

I want to login on server with my ssh keys in user MYUSER and, then execute some commands using shell module on server via root user
:NOTE: I have root password to provide in inventory. 
Below is the inventory that I created. (^ ^) I know it is wrong. 

```
test-web.example.com ansible_ssh_host=X.X.X.X ansible_ssh_user=MYUSER ansible_become_method=su ansible_become_user=root ansible_become_pass=Xjdt@mdaj12

```

Execution:

```
[root@server ansi]# ansible --private-key /root/MASTER/audit/key.key -i test -m shell -a "whoami" all
test-web.example.com | SUCCESS | rc=0 >>
MYUSER 

```

How can I make this work?