hello,
one option is to pass current user's login name from Sharepoint Online to button click handler and then to Azure function. This is how login name of current user can be retrieved in javascript:
var ctx = SP.ClientContext.get_current();
var user = web.get_currentUser();
ctx.load(user);
ctx.executeQueryAsync(
Function.createDelegate(this, function(sender, args) {
// pass user.get_LoginName() to AF
}),
Function.createDelegate(this, function(sender, args) {
// handle error
})
);
and then inside Azure function update Modified By field like that:
var ctx = ...
var user = ctx.Web.EnsureUser(loginName);
ctx.Load(user);
ctx.ExecuteQuery();
var item = ...
item["Editor"] = user;
item.Update();
ctx.ExecuteQuery();