var ss = SpreadsheetApp.getActiveSpreadsheet();
var sheet = ss.getSheets()[0];

var cell = sheet.getRange("B2");
// Sets borders on the top and bottom, but leaves the left and right unchanged
// Also sets the color to "red", and the border to "DASHED".
cell.setBorder(true, null, true, null, false, false, "red", SpreadsheetApp.BorderStyle.DASHED);
//.setBorder(top, left, bottom, right, vertical, horizontal, color, style)