Azure Ad b2c : change current email pattern validation on signup

I have an application running on azure ad b2c.

I have requirement to restrict email to only one domain. for this I have to change pattern in Email claim.

What I have tried it is I changed it in basepolicy and uploaded it. I got some new issues while signin.because that base policy was not matching the requirement of current signup-signin policy. I have downloaded it from custom policy starter packs.

Is there any another way to do this? OR any way to change the existing signup-signin policy.