We have just introduced an update that allows you to select a crop type from a list when creating usages.
These are standard crop types which can be used for official pesticide checks with a future update.
You can add a crop type to a field-use that you already have by:
Farm settings > Field Usages > Edit
Add a crop type in the box
On mobile find Farm Settings on the left-hand side menu.
On web find Farm Settings in web by tapping the cog in the top right.