A sample workflow for Umbraco forms that allows the editor to select a custom redirect URL. Great to combine with workflow conditions to give dynamic redirects based on form fields. Created for Umbraco v13, not tested in later versions.

ConditionalRedirectWorkflow.cs

using Umbraco.Forms.Core;
using Umbraco.Forms.Core.Enums;
using Microsoft.Extensions.Logging;
using Microsoft.AspNetCore.Http;
using Umbraco.Forms.Core.Attributes;
using Umbraco.Cms.Core;
using Umbraco.Extensions;
using Microsoft.Extensions.DependencyInjection;

namespace Example.Forms.Workflows
{
    public class ConditionalRedirectWorkflow : WorkflowType
    {
        private readonly IHttpContextAccessor _httpContextAccessor;
        private readonly IServiceProvider _serviceProvider;
        private readonly ILogger<ConditionalRedirectWorkflow> _logger;

        [Setting("Selected Page", Description = "Choose the page within the site to redirect to after submit", View = "Pickers.Content")]
        public string SelectedPage { get; set; }
        [Setting("Url", Description = "Or enter the URL manually (will override the Selected Page if entered)", View = "TextField")]
        public string ManualUrl { get; set; }

        public ConditionalRedirectWorkflow(IHttpContextAccessor httpContextAccessor, IServiceProvider serviceProvider, ILogger<ConditionalRedirectWorkflow> logger)
        {
            _httpContextAccessor = httpContextAccessor;
            _serviceProvider = serviceProvider;
            _logger = logger;

            this.Id = new Guid("4f88fb22-c8cc-4e41-975c-886859d36a8f");
            this.Name = "Conditional Redirect Workflow";
            this.Description = "Redirect to a specific destination page, can be used with conditional statements to dynamically redirect users.";
            this.Icon = "icon-filter-arrows";
            this.Group = "Services";
        }

        public override Task<WorkflowExecutionStatus> ExecuteAsync(WorkflowExecutionContext context)
        {
            string redirectUrl = "";
            if (!String.IsNullOrWhiteSpace(ManualUrl))
            {
                redirectUrl = ManualUrl;
            }
            else if(Int32.TryParse(SelectedPage, out int pageId))
            {
                try
                {
                    using (var scope = _serviceProvider.CreateScope())
                    {
                        IPublishedContentQuery _contentQuery = scope.ServiceProvider.GetService<IPublishedContentQuery>();
                        var content = _contentQuery.Content(pageId);
                        if (content != null)
                        {
                            redirectUrl = content.Url();
                        }
                    }
                }
                catch(Exception ex)
                {
                    _logger.LogError(ex, "Error fetching content during Conditional Redirect Workflow");
                    return Task.FromResult(WorkflowExecutionStatus.Failed);
                }
            }

            // If we have a redirect URL then push it into the context to be picked up later in the chain
            if (!String.IsNullOrWhiteSpace(redirectUrl))
            {
                _httpContextAccessor.HttpContext.Items[Umbraco.Forms.Core.Constants.ItemKeys.RedirectAfterFormSubmitUrl] = redirectUrl;
                return Task.FromResult(WorkflowExecutionStatus.Completed);
            }
            else
            {
                _logger.LogWarning("Unable to redirect workflow with ManualUrl: {0} / Url: {1}", ManualUrl ?? "", SelectedPage ?? "");
                return Task.FromResult(WorkflowExecutionStatus.Failed);
            }
        }

        public override List<Exception> ValidateSettings()
        {
            var ex = new List<Exception>();
            if(String.IsNullOrWhiteSpace(ManualUrl) && String.IsNullOrEmpty(SelectedPage))
            {
                ex.Add(new Exception("You must select at least one; Selected Page or Url"));
            }
            return ex;
        }
    }
}

FormsComposer.cs

using Ableton.Core.Forms.Workflows;
using Umbraco.Cms.Core.Composing;
using Umbraco.Cms.Core.DependencyInjection;
using Umbraco.Forms.Core.Providers;

namespace Example.Forms.Workflows
{
    public class FormsComposer : IComposer
    {
        public void Compose(IUmbracoBuilder builder)
        {
            builder.WithCollectionBuilder<WorkflowCollectionBuilder>()
                .Add<ConditionalRedirectWorkflow>();
        }
    }
}