Please can you list the steps you followed?

It seems you set up a redirect instead of a CNAME record as shown in the documentation https://docs.railway.com/guides/public-networking#adding-a-root-domain-with-www-subdomain-to-cloudflare

Im not using thie method of redirect www non www.
But even i have configured now that way... still making the same redirect to : docker-image-production-xxxxxxx.up.railway.app

[ Cloudflare ]
1 : I setup DNS ( api.mywebsite.com ) as CNAME with ( proxy Orange On ) to xxxxxx.up.railwayapp.

2 : SSL/TLS type : Full, with Automatic https rewrites ( off ) and always use https ( off )

3 : No page rules or redirect for primary domain.

[ Railway ]
1 : I register only the : api.mywebsite.com and i get the url xxxxx.up.railway to put on my Cloudflare.

2 : Thats all what i have done!

How is the correct tutorial to ( no more redirect ) to works right as i want, acessing from my custom domain : api.mywebsite.com ?

Hello,

You need to redeploy wordpress after adding a custom domain.

Best,
Brody

Thank you!
This solved my problem!