Tend gives you two options for creating fields:
Input Field: Creates a field shape based on the number of beds and the bed length and width. This option is better when your field is rectangular and has uniform bed lengths.
Drawn Field: Allows you to create your field shape by adding points along the perimeter of your field based on the satellite imagery on the map. This option is better when your field shape is not rectangular, having curved or non-parallel sides.
Please see the following articles for more information: