store the value you want to select into a variable.
Edit the Document Object (Ctrl-Alt-D), goto Tab Triggers, select "OnOpen" in the "Document Event Triggers" box, Click on "Edit Actions"
There you could click "Add"
1. Select Action Type = Selection and Action=Clear All, click on OK
2. Select Action Type = Selection and Action=Select in Field, click on OK now enter in the fields on the right in the Box "Field" the Name of the Field a value should be selected in tne box "Seatch String" enter "=VariableName" (where "VariableName" is the Name of the Variable you have stored the value within the script
Thats results that when the Document is opend, all values deselected and the field is preselected with the value of the variable.