Hey! I found out your question was interesting, so here is my answer :Option 1: Wildcard Subdomains (What I Use)Just add *.myapp.com in Railway → Settings → Custom DomainsDNS Setup:*.myapp.com → CNAME → <your-railway-domain>.up.railway.app *.myapp.com → CNAME → authorize.railwaydns.netImportant: If you're using Cloudflare, make sure authorize.railwaydns.net is DNS-only (gray cloud, not proxied).In my app:const host = req.headers.host; // "customer1.myapp.com" const tenant = host.split('.')[0]; // "customer1"That's it! Now customer1.myapp.com, customer2.myapp.com, etc. all work without touching Railway again.Option 2: True Custom Domains (Their Own Domain)If customers want dashboard.customer.com pointing to your app, you'll need to use Railway's API:const addCustomDomain = async (domain) => { await fetch('https://backboard.railway.app/graphql/v2', { method: 'POST', headers: { 'Authorization': `Bearer ${process.env.RAILWAY_API_TOKEN}`, 'Content-Type': 'application/json', }, body: JSON.stringify({ query: `mutation customDomainCreate($input: CustomDomainCreateInput!) { customDomainCreate(input: $input) { id domain status } }`, variables: { input: { domain, serviceId: process.env.RAILWAY_SERVICE_ID, environmentId: process.env.RAILWAY_ENVIRONMENT_ID } } }) }); };The flow: customer enters their domain → my backend calls Railway API → customer adds the CNAME → Railway handles SSL automatically.My recommendation: Start with wildcards. It's zero maintenance and scales perfectly. Only add the API integration if customers really need their own branded domains.Check out the docs here: https://docs.railway.com/guides/public-networking#wildcard-domainsHope this helps!

Sorry I am late to the party but bottom line is this,railway cannot viably support dynamic custom domains for multi tenant SaaS.Even with the api approach mentioned , Railway caps you at 1 domain on Free/Trial, 2 on Hobby, and 20 on Pro per service. The Pro limit can be raised on request, but it requires manually reaching out to Railway. That's not a scalable foundation,you'd be filing support requests every time you onboard a batch of customers, and there's no guarantee of how high they'll raise it.So forget the Railway API for this use case. The right tool is Cloudflare custom hostnamesThis is what Vercel, Netlify, and Render actually use internally. The idea is simple: Railway only ever sees one domain (app.myapp.com). Cloudflare handles every customer's hostname in front of it ,at unlimited scale.dashboard.customer.com → CNAME → proxy.yoursaas.com (Cloudflare) ↓ app.myapp.com (Railway)When a customer adds their domain in your UI, you call the Cloudflare API instead of Railway's:javascriptawait fetch(`https://api.cloudflare.com/client/v4/zones/${ZONE_ID}/custom_hostnames`, { method: 'POST', headers: { 'Authorization': `Bearer ${CF_TOKEN}`, 'Content-Type': 'application/json' }, body: JSON.stringify({ hostname: 'dashboard.customer.com', ssl: { method: 'http', type: 'dv', settings: { min_tls_version: '1.2' } } }) });Cloudflare provisions SSL for the customer's domain automatically, and your app reads the Host header to identify the tenant exactly as before. Railway never needs another domain registered