This script will look for the last row then it will copy and paste the data to a number of rows below. In fact, it will do like when I do a manual scroll/drag down by copying the selected cells then pasting them below by dragging down my mouse.

 

When I use the script ?

When I have to extend my data by copying from the last row to numbers of row down below.

 

How to create the script ?

Read How to create, edit and select an Office Script in an excel report

 

How to create the button to associate it with the script ?

Read How to create a button and associated it to an Office Script in an excel report

 

How is/are the script(s) ?

Copy the code below and paste it into your script. You will see my comments in green if exist so follow the help to adapt to your need.

function main(workbook: ExcelScript.Workbook) {
  // change sheet1 by yours  
  let sheet = workbook.getWorksheet("Sheet1");
let usedLastRow = sheet.getUsedRange().getRowCount();
  // change column A and C by yours and 5 by the number of row to paste
  // change usedLastRow by row number if to insert new rows in a specific row i.e row 4 -> ("A4:C4").autoFill(`A4:C${4 + 5}`
  sheet.getRange(`A${usedLastRow}:C${usedLastRow}`).autoFill(`A${usedLastRow}:C${usedLastRow + 5}`, ExcelScript.AutoFillType.fillDefault);
}              
              

The same result with another script:

function main(workbook: ExcelScript.Workbook) {
  // change sheet1 by yours   
  let sheet = workbook.getWorksheet("Sheet1");
  // change column A and C by yours
  let usedLastRowa = sheet.getRange("A:A").getUsedRange().getRowCount();
  let usedLastRowc = sheet.getRange("C:C").getUsedRange().getRowCount();
  // change 5 by the number of row to paste
  // change usedLastRowa by row range if to insert new rows in a specific row i.e row 4 -> ("A4:C4").autoFill(`A4:C${4 + 5}`
  sheet.getRange(`${usedLastRowa}:${usedLastRowc}`).autoFill(`${usedLastRowa}:${usedLastRowc + 5}`, ExcelScript.AutoFillType.fillDefault);
}              
              

This script is not related to autofill but I share it in case, it copies the last row to the new one (1 new row only):

function main(workbook: ExcelScript.Workbook) {
  // change sheet1 by yours
  let sheet = workbook.getWorksheet("Sheet1");
  let lastRowAddress = sheet.getUsedRange().getLastCell().getAddress();
  let lastRow = sheet.getRange(lastRowAddress).getEntireRow().getUsedRange();
  lastRow.getOffsetRange(1, 0).copyFrom(lastRow);
}              
              

This script is for a table:

function main(workbook: ExcelScript.Workbook) {
  // change sheet1 and table1 by yours
  let sheet = workbook.getWorksheet("Sheet1");
  let table = sheet.getTable("Table1");
  let countrow1 = table.getRowCount();
  let countrow2 = countrow1 + 1;
  let myarray: (string | number | boolean)[][] = [];
  // if your table has more than 3 columns, add ,"" i.e 4 column table -> ["", "", "", ""]
  const tabcolumn = ["", "", ""];
  //change 5 by number of row to add
  for (let i = 0; i < 5; i++) { myarray.push(tabcolumn) }
  //change countrow1 by row number if to insert new rows in a specific row i.e for row 4 put 3 -> (3, myarray)
  table.addRows(countrow1, myarray);
  //change 5 by number of row to add
  //if row number, replace countrow2 by row number i.e row 4 -> ("A4:C4").autoFill(`A4:C${4 + 5}`
  sheet.getRange(`A${countrow2}:C${countrow2}`).autoFill(`A${countrow2}:C${countrow2 + 5}`, ExcelScript.AutoFillType.fillDefault);
}              
              

Again, not related to autofill, it copies the last row to the new one for a table (1 new row only):

function main(workbook: ExcelScript.Workbook) {
    // change sheet1 and table1 by yours
    let sheet = workbook.getWorksheet("Sheet1");
    let table = sheet.getTable("Table1");
    let lastRowAddress = sheet.getUsedRange().getLastCell().getAddress();
    let lastRow = sheet.getRange(lastRowAddress).getEntireRow().getUsedRange();
    // if your table has more than 3 columns, add ,"" i.e 4 column table -> [["", "", "", ""]]
    let tabcolumn = [["", "", ""]];
    table.addRows(-1, tabcolumn);
    lastRow.getOffsetRange(1, 0).copyFrom(lastRow);
}