Please make sure you are in the Classic Interface.
To add custom or existing fields to specific forms:
From the main dashboard drop-down menu, click Settings, then Customizations.
Click Customize the System, which will bring up a new window.
In the left panel, expand Entities, then expand the entity you want to add a field to (e.g. Account) and then click Forms under the chosen Entity.
Click on the form you want to add a field to (e.g. Account Quick Create), which will open a new window.
From the Field Explorer on the right-hand side, find the field you want to add to the form and drag and drop them into the location you want on the form.
Select the field and click Change Properties in the top menu bar.
In the pop-up window, ensure the Visible by default box is checked in the display tab and click OK.
Click Save and then Publish.
You can now close the pop-up window to get back to the customization dashboard.