What you are looking for is a conditional dropdown. This can be done if your Subcategory dropdown is a related field (Subcategory is a parent to whatever table you are in). In that case, make a subcategories table and have 2 fields. 'Subcategory' and 'Category this Subcategory is part of'.
Then on your form, edit the related subcategory field and check conditional dropdown, where 'Category in this table' matches 'Category this Subcategory is a part of' on the related field.
Typically, this is actually done with a category table AND a subcategory table. Categories have many subcategories. Subcategories have many of whatever your main table is. This step isnt necessary for the conditional dropdown to work (your category dropdown could be a typical multiple choice field), but it's more common to have both the category nd subcategory to be related fields to additional tables.
