This apex code is to create custom field.

CreateCustomField.cls

        String sessionId = UserInfo.getOrganizationId()+''+UserInfo.getSessionId().SubString(15);

        List<String> objNames = new List<String>{'ConsumptionRate','ConsumptionSchedule','ProductConsumptionSchedule'};

        for(String objectapiname: objNames){
            String fieldapiname = 'External_Id';//replace with your field name
            String fieldlabel = 'External Id';//replace with your field label
            String fielddescription = 'External Id required for migration fro records between environments. Don\'t touch this field.';
            String inlineHelpText = 'External Id required for migration fro records between environments. Don\'t touch this field.';

            HttpRequest requestinside = new HttpRequest();
            requestinside.setHeader('Authorization', 'Bearer ' + sessionId);
            requestinside.setHeader('Content-Type', 'application/json');
            requestinside.setEndpoint(URL.getSalesforceBaseUrl().toExternalForm()+'/services/data/v41.0/tooling/sobjects/CustomField/');
            requestinside.setMethod('POST');
            String fieldDef = '{"Metadata" : ';
            String metadef = '"type" : "Number","description" : "'+fielddescription+'", "inlineHelpText" :"'+ inlineHelpText +'" ,"precision" : 18,"scale": 0,"label" : "'+fieldlabel+'","required" : false,"unique": true,"externalId": true';
            fieldDef += '{'+metadef+'},';
            fieldDef += '"FullName" : "'+objectapiname+'.'+fieldapiname+'__c"}';
            system.debug(fieldDef);
            requestinside.setBody(fieldDef);
            Http httpObj = new Http();
            HTTPResponse res = httpObj.send(requestinside);
            System.debug(res.getBody());
        }