Not just integrate form with google spreadsheet but the input form can fit with custom spreadsheet ?

Not just integrate form with google spreadsheet but the input form can fit with custom/own spreadsheet ?

I'm not sure if understood your question, but seems like you're trying to integrate your form with Google Spreadsheets, but you're looking for a way where you can select the spreadsheet where the data should be sent to. 

Currently this is not possible to do, you can only integrate your form with Google Spreadsheet, but over a new spreadsheet, it means that the spreadsheet is created in the process of integrate your form. 

You may try with this Zap: https://zapier.com/zapbook/jotform/google-docs/. 

We currently have an integrationg with Zapier so you only need to register and use this zap, it seems that this can achieve what you want and allow you to integrate your form with an existing spreadsheet.