Code to add the author / byline to a modern page (immediately below the title)

AddModernPageByline

/// <summary>
/// Sets the Author Byline for a modern page
/// </summary>
/// <remarks>
/// While there is a _AuthorByline field on the page list item and an AuthorBylineId and AuthorByline properties on the ClientSidePage,
/// these currently cannot be used to set the author byline.  For that we have to use LayoutWebpartsContent on the list item.
/// </remarks>
/// <param name="context"></param>
/// <param name="page"></param>
/// <param name="userName"></param>
/// <param name="pageName"></param>
public void AddModernPageByline(ClientContext context, ClientSidePage page, string userName, string pageName)
{
	try
	{
		// Only add the author byline if we can get the actual creator - if they don't resolve, then skip this
		User authorBylineUser = GetUser(context, userName);
		if (authorBylineUser != null)
		{
			// We have the actual creator resolved in SharePoint - try to get the LayoutWebPartsContent
			// Load the LayoutWebpartsContent page list item property - this is what needs to contain the author byline
			context.Load(page.PageListItem, p => p["LayoutWebpartsContent"]);
			context.ExecuteQuery();
			object layoutWebPartsContent = page.PageListItem["LayoutWebpartsContent"];
			if (layoutWebPartsContent != null)
			{
				// The LayoutWebPartsContent is HTML that we need to parse
				HtmlDocument doc = new HtmlDocument();
				doc.LoadHtml(layoutWebPartsContent.ToString());
				HtmlNode node = doc.DocumentNode.SelectSingleNode("/div/div");
				string controlDataAttribute = node.GetAttributeValue("data-sp-controldata", "");
				if (!String.IsNullOrEmpty(controlDataAttribute))
				{
					// This attribute has HTML encoded JSON - lets get the JSON out
					string controlDataAttributeDecoded = System.Web.HttpUtility.HtmlDecode(controlDataAttribute);
					JObject controlDataJSON = JObject.Parse(controlDataAttributeDecoded);

					// Now we need to update that JSON - it needs to be a JSON array containing the id, upn, name, and role for the user (we believe role can be blank)
					JArray authorBylineJArray = JArray.Parse(string.Format("[{{ \"id\": \"{0}\", \"upn\": \"{1}\", \"name\": \"{2}\", \"role\": \"\" }}]",
							authorBylineUser.LoginName, authorBylineUser.Email, authorBylineUser.Title));

					// Set the authors property - yes, there is an authorByline property, but it appears that authors is the one to set
					controlDataJSON["properties"]["authors"] = authorBylineJArray;

					// Now that the JSON is in order, serialize it, Html encode it, and set it to our HTML attribute in the doc
					string updatedControlDataAttributeDecoded = JsonConvert.SerializeObject(controlDataJSON, Formatting.Indented);
					string updatedControlDataAttribute = System.Web.HttpUtility.HtmlEncode(updatedControlDataAttributeDecoded);
					node.SetAttributeValue("data-sp-controldata", updatedControlDataAttribute);

					// Now that the doc has the updated author byline, put that back into the LayoutWebpartsContent field and save
					page.PageListItem["LayoutWebpartsContent"] = doc.DocumentNode.OuterHtml;
					page.PageListItem.Update();
					context.ExecuteQuery();
				}
			}
		}
	}
	catch (Exception ex)
	{
		// TODO - log or throw...
		string errMsg = string.Format("Unable to set Author Byline of page '{0}' to '{1}'.  {2}", pageName, userName, ex.Message);
	}
}

private User GetUser(ClientContext clientContext, string userName)
{
	User returnValue = null;

	try
	{
		User newUser = clientContext.Web.EnsureUser(userName);
		clientContext.Load(newUser);
		clientContext.ExecuteQuery();

		if (newUser != null)
		{
			returnValue = newUser;
		}
	}
	catch
	{
		// TODO - log or throw...
		string errMsg = string.Format("User '{0}' not found.", userName);
	}

	return returnValue;
}