Use force:createRecord to Prepopulate Fields on the Record Create Panel

The force:createRecord event supports a new attribute, defaultFieldValues. The defaultFieldValues attribute lets you prepopulate the create record form with default or calculated field values. Prepopulated values can accelerate data entry, improve data consistency, and otherwise make the process of creating a record easier. Specify default field values as name-value pairs in a JavaScript object.
This example displays the record create panel for a contact with two fields prepopulated.
var createAcountContactEvent = $A.get("e.force:createRecord");
createAcountContactEvent.setParams({
    "entityApiName": "Contact",
    "defaultFieldValues": {
        'Phone' : '415-240-6590',
        'Account' : '001xxxxxxxxxxxxxxx'
    }
});
createAcountContactEvent.fire();