There are two paths for adding a JMCF custom field - through the My Custom Fields page or through the native Jira Custom fields page.
To create a new field through My custom fields:
Log into your Jira instance as an Administrator.
Click Settings ⚙️ in the upper right corner.
Select Apps.
In the left-hand panel, click Jira Misc Custom Fields.
Select My custom fields.
Click New custom field in the upper right corner.
Select the JMCF field type and click Next.
Follow the steps in the next section to configure your custom field.
To create a new JMCF custom field through the Jira custom fields page:
Log into your Jira instance as an Administrator.
Click Settings ⚙️ in the upper right corner.
Select Issues.
In the left-hand panel, click Custom fields.
Click Create custom field in the upper right corner.
In the Select a Field Type window, click Advanced.
Select the JMCF field type you want to add and click Next.
Give the custom field a name and, optionally, a description.
Click Create.
Follow the steps in the next section to configure your custom field.
Note: If you create a JMCF custom field using the native Jira custom field process, the new custom field will not calculate until its Contexts have been updated; this limitation applies even when the custom field is configured for the Default context.