After getting this email you need to activate the user request. This activation is done by firing a POST call on URL
http://<HOST>:<PORT>/sap/opu/odata/IWBEP/USERREQUESTMANAGEMENT/UserRequestActivationRequestCollection('REQUESTID')
Here in the payload of the post you can pass the password you want.
Refer to http://<HOST>:<PORT>/sap/opu/odata/IWBEP/USERREQUESTMANAGEMENT/$metadata and see under entity UserRequestActivationRequest. You will find property "Password".