@Hiroki Miyashit ,

What do you mean by input form, Are you talking about some record of table?

BTW here is the code to send the approval to particular group, you need to create a UI action server side and write this code mentioned below:

var approval = new GlideRecord('sysapproval_group');
approval.initialize();
approval.assignment_group = //pass the sys_id of group to whom you want to send Approval request
approval.approval = 'requested';
approval.parent = current.sys_id;
approval.insert();
action.setRedirectURL(current);

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