As you are creating issue for JSM via the Create button via the project UI, there is no actual configuration can be setup to default request type based on my knowledge.
However, here is a workaround for you to consider -
Create a request type named it to "A something..." and don't expose it to any group (meaning it is hidden from the portal UI). The request type name is sorted by alphabetic order, so this form will show up as the first choice. Keep in mind, one can still click on the dropdown to select other request type.
Best, Joseph Chung Yin
Jira/JSM Functional Lead, Global Infrastructure Applications Team