https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_intro_get_dev_account.htm

https://developer.salesforce.com/docs/atlas.en-us.dev_lifecycle.meta/dev_lifecycle/creating_dev_sandbox.htm

Is it a way to create free sandbox to test installation app exchange ?

I'm really need to create global action which almost will looks like Opportunity -> 'New Event'.
I'm using visualforce for that.
Can I find it somewhere ?

And one more thing. I need to add `global action` widget to Opportunity->Activity
How can I make it or what should I set in `publisher layouts` ?

public with sharing class customOpportunityEvent {

    public Event event{get;set;}


    public customOpportunityEvent() {
        event = new Event();
    }

    public void fillConferenceInfo() {
        event.Subject = 'some Invitation';
        event.Location = 'location';
        event.Description = 'hi there';
    }

}

I need to get something like

on my visualforce page.

how can I create such component ?

I need a custom opportunity event hendler. 
behavior: on user click on button, request calls remote server and fill event location and description fields.
As I understand  it is impossable to integrate such button to existing `Opportunity`->`New Event`
that's why I've created new visualforce page for `quick action` with set of fields which similar to `New Event` component

But what should I do to save opportunity event ?

I need to save custom data set or value somewhere in salesforce. that value wil relates just to current user. So I need something like env variables or global user storage.

If yes. is it possible to make through api access? 

Will happy if someone can help me with that issue.

public with sharing class customOpportunityEvent {

    public Event event{get;set;}


    public customOpportunityEvent() {
        event = new Event();
    }

    public void fillConferenceInfo() {
        event.Subject = 'some Invitation';
        event.Location = 'location';
        event.Description = 'hi there';
    }

}

I need to save custom data set or value somewhere in salesforce. that value wil relates just to current user. So I need something like env variables or global user storage.

If yes. is it possible to make through api access? 

Will happy if someone can help me with that issue.