If you are in note entry mode. Enter the first note, change the duration to what you want for the second note and press the + sign. If you are not in note entry mode and want to add a tie to an existing note, use note entry mode to enter the second note with the same pitch as the first (including sharp or flat) if the note is not there already. Exit note entry mode if you are in it. Click the first note and press the + sign.
Comments
If you are in note entry mode. Enter the first note, change the duration to what you want for the second note and press the + sign. If you are not in note entry mode and want to add a tie to an existing note, use note entry mode to enter the second note with the same pitch as the first (including sharp or flat) if the note is not there already. Exit note entry mode if you are in it. Click the first note and press the + sign.
see https://musescore.org/en/handbook/tie