Select item from a ComboBox

Assuming the combo's first column, SubTopicID, is also the combo's "bound column" property, it is used as the column's .Value property. That means you only need to assign a value to .Value in order to select the matching combo row.

Me.cmbSubTopic.Value =  rsST.Fields("SubTopicID").Value

That approach is simple, but I'm uncertain whether it is the appropriate solution for your situation. We don't know anything about your rsST recordset --- I presumed the SubTopicID field in the recordset's current row is the value you want selected in the combo. If I misunderstood that point, we need to figure out something different.

If the combo is bound to a field in the form's record source, this suggestion would also change the stored value. If you don't want that, "unbind" the combo --- in other words, make its Control Source property blank.