Sandbox default value from LOV

Hi, is there a way to default the Category field on the Create Subledger Journal Entry screen (General Accounting > Journals > Create Subledger Journal Entry)

Currently, this field is a LOV which displays the Journal Category. We want this field to default to "Adjustment" or enable users to select Adjustment only from the LOV.

We have investigated the Page Composer in the Sandbox but cannot determine how to default this field.