PowerApps Set Dropdown default value formula

I have PowerApps form with dropdown box is Priority, when user open the form , by default it will load dropdown value is Low.

Here you are:

LookUp(Choices(ITHelpDesk.'Priority'),Value="Low")

How do I get Current User Login detail from MS power Apps

You can use the Office 365 Users Connector to get this information. 

Current User

Office365Users.MyProfile().Department

Office365Users.MyProfile().DisplayName

Office365Users.MyProfile().GivenName

Office365Users.MyProfile().Id

Office365Users.MyProfile().JobTitle

Office365Users.MyProfile().Mail

Office365Users.MyProfile().MailNickname

Office365Users.MyProfile().Surname

Office365Users.MyProfile().TelephoneNumber

Office365Users.MyProfile().UserPrincipalName

Office365Users.MyProfile().AccountEnabled

To show information about another user:
Office365Users.UserProfile(name@company.com).Department

Office365Users.UserProfile(name@company.com).DisplayName
Office365Users.UserProfile(name@company.com).GivenName
Office365Users.UserProfile(name@company.com).Id
Office365Users.UserProfile(name@company.com).JobTitle
Office365Users.UserProfile(name@company.com).Mail
Office365Users.UserProfile(name@company.com).MailNickname
Office365Users.UserProfile(name@company.com).Surname
Office365Users.UserProfile(name@company.com).TelephoneNumber
Office365Users.UserProfile(name@company.com).UserPrincipalName
Office365Users.UserProfile(name@company.com).AccountEnabled

To show information about another user's manager:Office365Users.Manager(name@company.com).Department
Office365Users.Manager(name@company.com).DisplayName
Office365Users.Manager(name@company.com).GivenName
Office365Users.Manager(name@company.com).Id
Office365Users.Manager(name@company.com).JobTitle
Office365Users.Manager(name@company.com).Mail
Office365Users.Manager(name@company.com).MailNickname
Office365Users.Manager(name@company.com).Surname
Office365Users.Manager(name@company.com).TelephoneNumber
Office365Users.Manager(name@company.com).UserPrincipalName
Office365Users.Manager(name@company.com).AccountEnabled