I manage to get the value through an API call:

{
  "jsonrpc": "2.0",
  "method": "item.get",
  "params": {
    "host": "{$MYHOST}",
    "search": {
      "key_": "oltPonTxOpticalPower.[{$MYHOSTPWID}]"
    }
  },
  "auth": "{$API_TOKEN}", "id":{$MYHOSTPWID}
}

But the HTTP Agent call is overloading my server.

Zabbix does not support custom macros in this context. This is also reflected in the documentation.

User macros in the formula will be expanded if used to reference a function parameter, item filter parameter, or a constant. User macros will NOT be expanded if referencing a function, host name, item key, item key parameter or operator.

Thanks. Can I use any other function, or aggregate function, to select only one host?

In this case, you can use the macro {HOST.HOST} or omit the host name. Both of these approaches will work the same way:

last(/{HOST.HOST}/oltPonTxOpticalPower.[472907776])

last(//oltPonTxOpticalPower.[472907776])

Thanks.
In this case, {HOST.HOST} don't work from me. I want to get the value from a key from another host. It is a parent host, in the case.
I can manage to get this value through API, via a HTTP Agent item.

{
  "jsonrpc": "2.0",
  "method": "item.get",
  "params":
{     "host": "{$MYHOST}",
    "search":
{       "key_": "oltPonTxOpticalPower.[{$MYHOSTPWID}]" 
   }
  },
  "auth": "{$API_TOKEN}", "id":{$MYHOSTPWID}
}

The parent host is defined by {$MYHOST} macro. I  have a host tag defined as {$MYHOST} too.

In this case, you need to explicitly specify the hostname. There are no other options.

You can also vote for this functionality in ticket ZBXNEXT-8616.

Can I use a LLD to concatenate the item?

No, LLD macros cannot be used to concatenate the item key in a calculated item formula. However, in item prototypes, at least one such macro is required in the item key parameter. Here is an example of a formula that contains references to item prototypes:

(last(//vfs.dev.read.time.rate[{#DEVNAME}])/(last(//vfs.dev.read.rate[{#DEVNAME}])+(last(//vfs.dev.read.rate[{#DEVNAME}])=0)))*1000*(last(//vfs.dev.read.rate[{#DEVNAME}]) > 0)