Hi,

Salesforce has released an update that allows you to use a custom URL and perhaps more importantly a private SSL certificate with your customer and partner communities.

http://releasenotes.docs.salesforce.com/en-us/summer14/release-notes/communities_custom_domain_URL.htm

Before this update users would see “force.com” as the base domain (e.g. https://mycompany.force.com) after they logged in.

Setting up the custom URL and SSL certificate is relatively painless. You will need to do the following;

Add the custom URL to Salesforce
Generate a Certificate Signing Request from Salesforce
Create a CNAME (Alias) record in your domain name registrar
Generate an SSL certificate in your domain name registrar
Upload the SSL certificate to Salesforce
Custom domains are supported only in non-sandbox instances. You can configure a custom domain in a sandbox instance and then migrate it to a production instance, but the custom domain is only active in production.

Try the steps as mentioned in the below blog,

https://www.thinkaholics.com/salesforce/how-to-add-a-custom-url-to-salesforce-communities/

For further reference, you can check the below article too,

https://help.salesforce.com/articleView?id=000336819&type=1&mode=1

I hope you find the above information is helpful. If it does, please mark as Best Answer to help others too.

Thanks.