To create a custom field, first go to Settings > General Settings > Custom Fields and then click on Configure. Click on the appropriate location for the custom field and then click on Add in the upper right corner.
When creating a custom field, you have several configuration options to choose from. The screen for configuring a custom field will have different options depending on what it is being created for (i.e. projects, orders, etc.). It will appear as shown below when creating a custom field for a project:
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