Notes

function rightmost(sheetName, rowNumber, offset) {
// Access the active spreadsheet
var spreadsheet = SpreadsheetApp.getActiveSpreadsheet();

try {
// Get the specified sheet
var sheet = spreadsheet.getSheetByName(sheetName);

// Get the data range for the specified row
var dataRange = sheet.getRange(rowNumber, 1, 1, sheet.getLastColumn());

// Get the values in the row
var values = dataRange.getValues()[0];

// Iterate through the values in reverse order
for (var i = values.length - 1, count = 0; i >= 0; i--) {
if (values[i] !== "" && values[i] !== null && values[i] !== undefined) {
count++;
if (count === offset + 1) {
// Log the result (you can modify this part based on what you want to do with the data)
var offsetCellContent = values[i];
Logger.log('Content of the cell ' + offset + ' left from the rightmost filled cell on row ' + rowNumber + ' in sheet ' + sheetName + ': ' + offsetCellContent);

// Return the content of the offset cell
return offsetCellContent;
}
}
}

// If all cells are empty, return null
return null;

} catch (error) {
// Handle any errors
Logger.log('Error: ' + error);
return "Script error";
}
}//