Create a new record from a parent record using a Smart Button
Scenario:
You want to create a shortcut button to create a child contact record on the account main form command bar (Rather than the + button on the contact subgrid). You also want to map the values using the default mappings.
Steps:
1. Install the Smart Buttons Solution
2. Open the Ribbon Workbench with a solution containing the Account entity (no sub-components)
3. Drag a Quick JS button, name the button something like 'Create New Contact'
4. Enter the following JavaScript
var entityFormOptions = {
"entityName":"contact",
"createFromEntity": {
"entityType":"account",
"id":context.data.entity.getId()}
};
// Open the form.
Xrm.Navigation.openForm(entityFormOptions);
It should look something like the following:

See more info on the openForm API - https://docs.microsoft.com/en-us/powerapps/developer/model-driven-apps/clientapi/reference/xrm-navigation/openform
You can use this API to also show the Quick Create Form, or open a Main Form Dialog